docs/ci: treat warnings as errors

When we moved building the docs to Meson, we accidentally dropped the -W
flag that we used to have. This lead to us no longer detecting certain
problems in the docs, which is unfortunate.

Let's bring this back gated by the werror meson-option, and wire that up
on the CI end.

Fixes: fdd204538b ("ci: build docs using meson")
Reviewed-by: Dylan Baker <None>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32831>

docs/meson.build

@@ -11,12 +11,18 @@ assert Version(hawkmoth.__version__) >= Version("0.16.0")
   error_message: 'hawkmoth >= 0.16.0 required'
 )
 
+sphinx_args = []
+if get_option('werror')
+  sphinx_args += '-W'
+endif
+
 html_docs = custom_target(
   'html',
   depfile: 'docs.d',
   depend_files: files('conf.py'),
   command: [sphinx, '-b', 'html', '-q',
             '-Ddepfile=@DEPFILE@',
+            sphinx_args,
             meson.current_source_dir(), '@OUTPUT@'],
   output: 'html',
   build_by_default: get_option('html-docs').enabled(),