[docs] Generate OwnTone documentation with MkDocs

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docs/smart-playlists.md

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+# OwnTone smart playlists
+
+
+To add a smart playlist to the server, create a new text file with a filename ending with .smartpl; 
+the filename doesn't matter, only the .smartpl ending does. The file must be placed somewhere in your
+library folder.
+
+
+## Syntax
+
+The contents of a smart playlist must follow the syntax:
+
+```
+"Playlist Name" { expression }
+```
+
+There is exactly one smart playlist allowed for a .smartpl file.
+
+
+An expression consists of:
+
+```
+field-name operator operand
+```
+
+Where valid field-names (with their types) are:
+
+* `artist` (string)
+* `album_artist` (string)
+* `album` (string)
+* `title` (string)
+* `genre` (string)
+* `composer` (string)
+* `comment` (string)
+* `path` (string)
+* `type` (string)
+* `grouping` (string)
+* `data_kind` (enumeration)
+* `media_kind` (enumeration)
+* `play_count` (integer)
+* `skip_count` (integer)
+* `rating` (integer)
+* `year` (integer)
+* `compilation` (integer)
+* `track` (integer)
+* `disc` (integer)
+* `time_added` (date)
+* `time_modified` (date)
+* `time_played` (date)
+* `time_skipped` (date)
+* `random` (special)
+
+Valid operators include:
+
+* `is`, `includes`, `starts with`, `ends with` (string)
+* `>`, `<`, `<=`, `>=`, `=` (int)
+* `after`, `before` (date)
+* `is` (enumeration)
+
+The `is` operator must exactly match the field value, while the `includes` operator matches a substring.
+The `starts with` operator matches, if the value starts with the given prefix,
+and `ends with` matches the opposite. All these matches are case-insensitive.
+
+Valid operands include:
+
+* "string value" (string)
+* integer (int)
+
+Valid operands for the enumeration `data_kind` are:
+
+* `file`
+* `url`
+* `spotify`
+* `pipe`
+
+Valid operands for the enumeration `media_kind` are:
+
+* `music`
+* `movie`
+* `podcast`
+* `audiobook`
+* `tvshow`
+
+
+Multiple expressions can be anded or ored together, using the keywords `OR` and `AND`. The unary not operator is also supported using the keyword `NOT`.
+
+
+It is possible to define the sort order and limit the number of items by adding an order clause and/or a limit clause after the last expression:
+
+```
+"Playlist Name" { expression ORDER BY field-name sort-direction LIMIT limit }
+```
+
+"sort-direction" is either `ASC` (ascending) or `DESC` (descending). "limit" is the maximum number of items.
+
+There is additionally a special `random` _field-name_ that can be used in conjunction with `limit` to select a random number of items based on current expression.
+
+
+## Examples
+
+```
+"techno" {
+   genre includes "techno"
+   and artist includes "zombie"
+}
+```
+
+This would match songs by "Rob Zombie" or "White Zombie", as well as those with a genre of "Techno-Industrial" or
+"Trance/Techno", for example.
+
+```
+"techno 2015" {
+   genre includes "techno"
+   and artist includes "zombie"
+   and not genre includes "industrial"
+}
+```
+
+This would exclude e. g. songs with the genre "Techno-Industrial".
+
+```
+"Local music" {
+  data_kind is file
+  and media_kind is music
+}
+```
+
+This would match all songs added as files to the library that are not placed under the folders for podcasts, audiobooks.
+
+```
+"Unplayed podcasts and audiobooks" {
+  play_count = 0
+  and (media_kind is podcast or media_kind is audiobook)
+}
+```
+
+This would match any podcast and audiobook file that was never played.
+
+```
+"Recently added music" {
+  media_kind is music
+  order by time_added desc
+  limit 10
+}
+```
+This would match the last 10 music files added to the library.
+
+```
+"Random 10 Rated Pop songs" {
+  rating > 0 and
+  genre is "Pop" and
+  media_kind is music
+  order by random desc
+  limit 10
+}
+```
+This generates a random set of, maximum of 10, rated Pop music tracks every time the playlist is queried.
+
+## Date operand syntax
+
+One example of a valid date is a date in yyyy-mm-dd format:
+
+```
+"Files added after January 1, 2004" {
+  time_added after 2004-01-01
+}
+```
+
+There are also some special date keywords:
+
+* `today`, `yesterday`, `this week`, `last week`, `last month`, `last year`
+
+These dates refer to the _start_ of that period; `today` means 00:00hrs of today, `this week` means current Monday 00:00hrs, `last week` means the previous Monday 00:00hrs, `last month` is the first day of the previous month at 00:00hrs etc.
+
+A valid date can also be made by applying an interval to a date. Intervals can be defined as `days`, `weeks`, `months`, `years`.
+As an example, a valid date might be:
+
+```3 weeks before today``` or ```3 weeks ago```
+
+
+Examples:
+
+```
+"Recently Added" {
+    time_added after 2 weeks ago
+}
+```
+
+This matches all songs added in the last 2 weeks.
+
+```
+"Recently played audiobooks" {
+    time_played after last week
+    and media_kind is audiobook
+}
+```
+
+This matches all audiobooks played since the start of the last Monday 00:00AM.
+
+All dates, except for `YYYY-DD-HH`, are relative to the day of when the server evaluates the smartpl query; `time_added after today` run on a Monday would match against items added since Monday 00:00hrs and evaluating the same smartpl on Friday would only match against added on Friday 00:00hrs.
+
+Note that `time_added after 4 weeks ago` and `time_added after last month` are subtly different; the former is exactly 4 weeks ago (from today) whereas the latter is the first day of the previous month.
+
+
+## Differences to mt-daapd smart playlists
+
+The syntax is really close to the mt-daapd smart playlist syntax (see
+http://sourceforge.net/p/mt-daapd/code/HEAD/tree/tags/release-0.2.4.2/contrib/mt-daapd.playlist).
+
+Even this documentation is based on the file linked above.
+
+Some differences are:
+
+* only one smart playlist per file
+* the not operator must be placed before an expression and not before the operator
+* `||`, `&&`, `!` are not supported (use `or`, `and`, `not`)
+* comments are not supported
+