docs: write basic meta-documentation

We were previously completely lacking documentation on how to write
documentation. This adds some very basic docs for that.

Maybe at some point this could deserve it's own article, but for now
this seems enough.

This also documents the group-consensus on preferring US English that
was reached on the mesa-dev mailing list[1].

[1]: https://lists.freedesktop.org/archives/mesa-dev/2021-March/225023.html

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9912>

docs/submittingpatches.rst

@@ -360,6 +360,21 @@ staging/year.quarter branch, not on the year.quarter branch, per the
 stable branch policy. Assigning the MR to release maintainer for said
 branch or mentioning them is helpful, but not required.
 
+Documentation patches
+---------------------
+
+Our documentation is written as `reStructuredText`_ files in the
+:file:`docs` folder, and built using `Sphinx`_.
+
+The preferred language of the documentation is US English. This
+doesn't mean that everyone is expected to pay close attention to
+the different English variants, but it does mean someome might
+suggest a spelling-change, either during review or as a follow-up
+merge-request.
+
+.. _reStructuredText: https://docutils.sourceforge.io/rst.html
+.. _Sphinx: https://www.sphinx-doc.org/
+
 Git tips
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