[ticket/12603] Remove hook system documentation.

The code for hooks still exists in Ascraeus for compatibility reasons, however,
it is no longer recommended that they be used. The events system should be
used instead.

PHPBB3-12603
Merge pull request #2503 from prototech/ticket/12603

[ticket/12603] Remove hook system documentation.

* prototech/ticket/12603:
  [ticket/12603] Remove hook system documentation.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12603] Remove hook system documentation.