docs: set language to english

docs: set language to english

It seems recent versions of Sphinx has started issuing a warning when
language isn't set to a recognized value. Since we're building with
warnings as errors on CI, this causes issues for people.

We're writing English docs, so we should set this to English. I probably
confused this setting with the highlight_language setting or something
when I set this up initially.

Fixes: 00cd1346bf0 ("docs: use sphinx")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6711
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17159>