Restore docs/PAX_CONTROL_GROUPS

Restore docs/PAX_CONTROL_GROUPS

This partially reverts 8d04b8198d4c0cca0118f731369ad7156f0726b6.
If we completely drop the file, users will get a 404. But this document
has been in place for a long time and is referred to in many other places,
incl. our old wiki at https://www.freedesktop.org/wiki/Software/.
The page already says that it's been replaced
("… Please consult this document only as a historical reference. …").
We should only remove it from the index (which
8d04b8198d4c0cca0118f731369ad7156f0726b6 did).

In general, let's be more careful about preserving link stability.
When we change something in a way that breaks URLs, we're creating
pain for users.