meson: build html-docs

This allows us to build our Sphinx-based documentation from the meson
build system.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11494>

meson.build

@@ -2304,6 +2304,12 @@ subdir('src')
 
 meson.add_devenv(devenv)
 
+sphinx = find_program('sphinx-build', version : '>= 4.3',
+                      required: get_option('html-docs'))
+if sphinx.found()
+  subdir('docs')
+endif
+
 summary(
   {
     'prefix': get_option('prefix'),