ci: implement farms handling trough files inside .ci-farms

- Disable farm with `git mv .ci-farm{,-disabled}/$farm_name`.
 - Re-enable farm with `git mv .ci-farm{-disabled,}/$farm_name`.

NEVER MIX FARM MAINTENANCE WITH ANY OTHER CHANGE IN THE SAME MERGE REQUEST!

Reviewed-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Acked-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23629>

docs/ci/index.rst

@@ -52,6 +52,23 @@ The three GitLab CI systems currently integrated are:
    LAVA
    docker
 
+Farm management
+---------------
+
+.. note::
+   Never mix farm maintenance with any other change in the same merge request!
+
+When the farm starts failing for any reason (power, network, out-of-space), it needs to be disabled by pushing separate MR with
+.. code-block:: console
+ 
+   git mv .ci-farm{,-disabled}/$farm_name`
+   
+After farm restore functionality can be enabled by pushing a new merge request, which contains
+.. code-block:: console
+
+   git mv .ci-farm{-disabled,}/$farm_name`
+
+
 Application traces replay
 -------------------------