docs: translations: use attribute to store current language

Akira Yokosawa reported [1] that the "translations" extension we added in
commit 7418ec5b151f ("docs: translations: add translations links when they
exist") broke the build on Sphinx versions v6.1.3 through 7.1.2 (possibly
others) with the following error:

Exception occurred:
File "/usr/lib/python3.12/site-packages/sphinx/util/nodes.py", line 624, in _copy_except__document
newnode = self.__class__(rawsource=self.rawsource, **self.attributes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: LanguagesNode.__init__() missing 1 required positional argument: 'current_language'
The full traceback has been saved in /tmp/sphinx-err-7xmwytuu.log, if you want to report the issue to the developers.

Solve this problem by making 'current_language' a true element attribute
of the LanguagesNode element, which is probably the more correct way to do
it anyway.

Tested on Sphinx 2.x, 3.x, 6.x, and 7.x.

[1]: https://lore.kernel.org/all/54a56c2e-a27c-45a0-b712-02a7bc7d2673@gmail.com/

Fixes: 7418ec5b151f ("docs: translations: add translations links when they exist")
Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Closes: https://lore.kernel.org/all/54a56c2e-a27c-45a0-b712-02a7bc7d2673@gmail.com/
Tested-by: Akira Yokosawa <akiyks@gmail.com> # Sphinx 4.3.2, 5.3.0 and 6.2.1
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215064109.1193556-1-vegard.nossum@oracle.com

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