[docs] Generate OwnTone documentation with MkDocs

- Copy existing README*.md and INSTALL.md files to docs folder
- Set up MkDocs to build documentation (mkdocs.yml)
- Add new index.md with a quick overview of OwnTone

docs/assets/extra.js

@@ -0,0 +1,32 @@
+/*
+ * Add click event handler for images with class="zoom"
+ */
+document.querySelectorAll('img.zoom').forEach(item => {
+    const p = item.parentElement;
+    if (!p.classList.contains('processed')) {
+        p.classList.add('processed');
+        if (p.querySelectorAll('img.zoom').length === p.children.length) {
+            p.classList.add('zoom-wrapper');
+        }
+    }
+    item.addEventListener('click', function () {
+        const img = document.getElementById('fullscreen-image-img');
+        img.setAttribute('src', this.getAttribute('src'));
+        img.setAttribute('alt', this.getAttribute('alt'));
+
+        const div = document.getElementById('fullscreen-image');
+        div.classList.replace('hidden', 'visible');
+    })
+});
+
+var div = document.createElement('div');
+div.classList.add('fullscreen-image-background', 'hidden');
+div.id = 'fullscreen-image';
+var img = document.createElement('img');
+img.id = 'fullscreen-image-img';
+div.appendChild(img);
+
+div.addEventListener('click', function () {
+    this.classList.replace('visible', 'hidden');
+});
+document.body.appendChild(div);