docs: Begin documenting ISL

This commit mostly just adds the framework required to scrape
documentation out of the ISL sources and headers.  The method chosen
here is a combination of doxygen and breathe (a sphinx extension for
doxygen integration).  I'll freely admit that doxygen is pretty terrible
but it seems like the best option we have available to us today.

Acked-by: Emma Anholt <emma@anholt.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11366>

docs/conf.py

@@ -38,7 +38,7 @@ sys.path.append(os.path.abspath('_exts'))
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['sphinx.ext.graphviz', 'formatting', 'redirects']
+extensions = ['sphinx.ext.graphviz', 'breathe', 'formatting', 'redirects']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -212,3 +212,11 @@ texinfo_documents = [
 # -- Options for Graphviz -------------------------------------------------
 
 graphviz_output_format = 'svg'
+
+# -- Options for breathe --------------------------------------------------
+breathe_projects = {
+    'mesa' : 'doxygen_xml',
+}
+breathe_default_project = 'mesa'
+breathe_show_define_initializer = True
+breathe_show_enumvalue_initializer = True