ci: move docs-stuff out of root .gitlab-ci.yml

ci: move docs-stuff out of root .gitlab-ci.yml

Unlike most other cases, we don't put the YAML-file in a ci-folder,
because we already have one for the CI-specific docs. So let's just
leave the YAML file directly in the docs-folder.

This should fix the problem that any docs-changes that touches the
CI-rules needs a full CI run just because of touching the root
.gitlab-ci.yml file. This causes needless friction and wastes CI
resources.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21953>