docs: remove doxygen support

It seems building the doxygen docs has bit-rotted over time, and now
generates a set of empty modules, apart from some basic descriptions.

Since Mesa is mostly implementing externally documented APIs, I don't
think it makes a whole lot of sense trying to fix this, and I think the
presence of these files might confuse users who try them out.

So let's just get rid of this.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10611>

docs/index.rst

@@ -106,7 +106,6 @@ and other operating systems.
    submittingpatches
    releasing
    release-calendar
-   sourcedocs
    dispatch
    gallium/index
    android

docs/sourcedocs.rst

@@ -1,18 +0,0 @@
-Source Documentation
-====================
-
-`Doxygen <http://www.doxygen.nl>`__ is used to automatically produce
-cross-referenced documentation from the Mesa source code.
-
-The Doxygen configuration files and generated files are not included in
-the normal Mesa distribution (they're very large). To generate Doxygen
-documentation, download Mesa from Git, change to the ``doxygen``
-directory and run ``make``.
-
-For an example of Doxygen usage in Mesa, see a recent source file such
-as
-`bufferobj.c <https://gitlab.freedesktop.org/mesa/mesa/-/blob/master/src/mesa/main/bufferobj.c>`__.
-
-If you're reading this page from your local copy of Mesa, and have run
-the doxygen scripts, you can read the documentation
-`here <../doxygen/main/index.html>`__