docs: scroll navigation to current active section

Add a custom JavaScript file to all HTML rendered docs output.

For now it only hosts a small code snipped which gets the current
active section link and bring it into view.
Needs to be triggered after DOM is initially loaded (which is still
before *all* resources like images, iframes, ... are necessarily
loaded), else the query cannot work.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/conf.py

@@ -229,6 +229,10 @@ html_favicon = 'images/favicon.ico'
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
+html_js_files = [
+    'custom.js',
+]
+
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied
 # directly to the root of the documentation.