Cleanup sphinx and readthedocs configurations

* Use dirhtml as default builder for readthedocs (clean URLs)
* The configuration variables are now sorted with respect to the Sphinx
  documentation
* Remove useless comments in conf.py
* Add new comments for each section, following Sphinx documentation
  order
* The code corresponding to videos have been moved from config to a
  dedicated extension.
* Use proper HTML theme options
* Exclude all files starting with `.`, `_` (sphinx convention)
* Use OpenSearch

_ext/videos.py

@@ -0,0 +1,68 @@
+"""
+    ReST directive for embedding Youtube and Vimeo videos.
+
+    There are two directives added: ``youtube`` and ``vimeo``. The only
+    argument is the video id of the video to include.
+    Both directives have three optional arguments: ``height``, ``width``
+    and ``align``. Default height is 281 and default width is 500.
+
+    Example::
+
+        .. youtube:: anwy2MPT5RE
+           :height: 315
+           :width: 560
+           :align: left
+
+    :copyright: (c) 2012 by Danilo Bargen.
+    :license: BSD 3-clause
+
+    From https://gist.github.com/ehles/bed012d78aad5d3cd6c35a49bef32f9f
+"""
+from __future__ import absolute_import
+from docutils import nodes
+from docutils.parsers.rst import Directive, directives
+
+def align(argument):
+    """Conversion function for the "align" option."""
+    return directives.choice(argument, ('left', 'center', 'right'))
+
+
+class IframeVideo(Directive):
+    has_content = False
+    required_arguments = 1
+    optional_arguments = 0
+    final_argument_whitespace = False
+    option_spec = {
+        'height': directives.nonnegative_int,
+        'width': directives.nonnegative_int,
+        'align': align,
+    }
+    default_width = 500
+    default_height = 281
+
+    def run(self):
+        self.options['video_id'] = directives.uri(self.arguments[0])
+        if not self.options.get('width'):
+            self.options['width'] = self.default_width
+        if not self.options.get('height'):
+            self.options['height'] = self.default_height
+        if not self.options.get('align'):
+            self.options['align'] = 'left'
+        return [nodes.raw('', self.html % self.options, format='html')]
+
+
+class GeneralVid(IframeVideo):
+    html = '<iframe src="%(video_id)s" width="%(width)u" height="%(height)u" frameborder="0" webkitAllowFullScreen mozallowfullscreen allowfullscreen class="responsive" referrerpolicy="no-referrer" scrolling="no"></iframe>'
+
+
+class Youtube(IframeVideo):
+    html = '<iframe src="https://www.youtube-nocookie.com/embed/%(video_id)s" \
+    width="%(width)u" height="%(height)u" frameborder="0" \
+    webkitAllowFullScreen mozallowfullscreen allowfullscreen \
+    class="responsive" referrerpolicy="no-referrer" scrolling="no"></iframe>'
+
+
+def setup(builder):
+    directives.register_directive('youtube', Youtube)
+    directives.register_directive('generalvid', GeneralVid)
+