docs: Document how to build and install Android drivers.

This is what I've been using so far.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7227>
diff --git a/docs/contents.rst b/docs/contents.rst
index 4e2e458..f7fd50d 100644
--- a/docs/contents.rst
+++ b/docs/contents.rst
@@ -64,6 +64,7 @@
    sourcedocs
    dispatch
    gallium/index
+   android
 
 .. toctree::
    :maxdepth: 1