When the template was made responsive, the choice was made to override
the `body_tag` fragment.
The `body_tag` block is a literal `<body>` which allows the theme developer
to set properties on the `body` tag.
See: sphinx-doc/sphinx#4140

Instead, use the `document` block as documented.
https://www.sphinx-doc.org/en/master/development/templating.html#blocks

Overriding the correct block places the `mobile-nav` section _after_ the
majority of the other HTML parts, which enables the correct function of
the `/` keyboard shortcut.

Resolves python#130
Closes python#131
Closes python#135

Signed-off-by: Mike Fiedler <[email protected]>