docs: translations: add translations links when they exist

Add a new Sphinx extension that knows about the translations of kernel
documentation and can insert links to the translations at the top of
the document.

It basically works like this:

1. Register a new node type, LanguagesNode.

2. Register a new transform, TranslationsTransform, that inserts a new
LanguageNode at the top of every document. The LanguageNode contains
"pending references" to translations of the document. The key here
is that these are pending (i.e. unresolved) references that may or
may not actually exist.

3. Register a 'doctree-resolved' event that iterates over all the
LanguageNode nodes. Any unresolved references are filtered out; the
list of resolved references is passed to the 'translations.html'
template and rendered as an HTML node (if HTML output is selected).

Testing: make htmldocs, make latexdocs with Sphinx v4.3.2 and Firefox.

v2:
- changed bar into a drop-down menu
- fixed language labels
- fixed hysteresis reported by Akira Yokosawa

Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231215123701.2712807-1-vegard.nossum@oracle.com

docs: Change <h4> style to use smaller font size than <h3>

The docs currently have <h3> and <h4> set to the same font size which
makes headings hard to distinguish. <h1> to <h3> already have entries in
sphinx-static/custom.css to shrink their size a bit from the alabaster
theme.

Add <h4> to custom.css and set it to be smaller than <h3>.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231212213137.98453-1-donald.hunter@gmail.com

Documentation: add tux logo

We already have the logo, let's use it.

Testing: make htmldocs

Cc: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231029074207.297663-1-vegard.nossum@oracle.com

docs: style toctree captions as headings

The rtd theme already styles toctree captions as headings, but the
alabaster theme doesn't. Add this in.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-2-vegard.nossum@oracle.com

docs: Add more information to the HTML sidebar

Add a new sidebar template that creates a more RTD-like "fisheye" view of
the current place in the document hierarchy. It is far from ideal, but
some readers may find it better for navigating through the documentation as
a whole.

Add some CSS trickery as well to make the table of contents less intrusive
when viewing the pages on a small screen.

Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: David Gow <davidgow@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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>

docs: improve the HTML formatting of kerneldoc comments

Make a few changes to cause functions documented by kerneldoc to stand out
better in the rendered documentation. Specifically, change kernel-doc to
put the description section into a ".. container::" section, then add a bit
of CSS to indent that section relative to the function prototype (or struct
or enum definition). Tweak a few other CSS parameters while in the
neighborhood to improve the formatting.

Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: tweak some Alabaster style parameters

This is just the beginning: tighten up the layout a bit to improve the
information density in the browser. Also reconfigure the page width in
terms of character units (em) rather than pixels, making it more
display-independent. To that end, add a custom.css file to
tweak Alabaster CSS settings.

Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>