docs: Fix config option for spelling filters

We've been using a custom spelling filter to make sure that "WireGuard"
is spelled correctly, with the proper case. As it turns out, passing
this filter to the Sphinx configuration, in the conf.py file, makes
Sphinx re-read all sources and re-write all output files, as can be
observed when running sphinx-build multiple times, without suppressing
the output:

    $ sphinx-build -M html . _build
    [...]
    updating environment: [config changed ('spelling_filters')] 472 added, 30 changed, 0 removed
    [...]

This is because in conf.py, we pass the filter directly as a function.
When Sphinx writes its environment.pickle file to keep track of the
configuration in use, it discards values that cannot be serialised,
including the filter, "<class 'cilium_spellfilters.WireGuardFilter'>",
of instance "type" [0]. So the value for the configuration option
"spelling_filters" is not saved, and as Sphinx believes that the
configuration has changed, it reads and rebuilds everything.

In fact, the issue has been reported before, and solved in the
spellchecker [1]. We need to set the configuration with a string instead
of the direct function object, and the extension is able to load it from
there. Let's adjust accordingly, to save cycles when building the docs
more than once.

[0] https://github.com/sphinx-doc/sphinx/blob/v7.1.2/sphinx/config.py#L323
[1] sphinx-contrib/spelling#40

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Documentation/conf.py

@@ -172,7 +172,7 @@
 spelling_exclude_patterns = ['_api/v1/*/README.md']
 
 # Add custom filters for spell checks.
-spelling_filters = [cilium_spellfilters.WireGuardFilter]
+spelling_filters = ["cilium_spellfilters.WireGuardFilter"]
 
 # Ignore some warnings from MyST parser
 suppress_warnings = ['myst.header']