docs: Don't wire font sizes for HTML output

The alabaster theme likes to provide explicit sizes for fonts, which
overrides the users's own browser settings and is guaranteed to displease
folks.  Set the font size to "inherit" so that the users browser settings
control the font size they get.  We can use the font_size configuration
option for the main body font (changing the size I'd already put there),
but the sidebar size can only be set via custom CSS.

Reported-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>