docs: make c:expr the default-role

Mesa is a C-project, so c:expr is a really handy role to default to.
This means that `foo` is a short-hand for :c:expr:`foo`.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19575>

docs/conf.py

@@ -91,6 +91,7 @@ todo_include_todos = False
 # python keywords highlit in literal blocks.
 highlight_language = 'none'
 
+default_role = 'c:expr'
 
 # -- Options for HTML output ----------------------------------------------