Add tombstones for removed collections

[felixfontein]

Uses the information added in ansible-community/ansible-build-data#459 to add tombstones for removed collections. This ensures that if you are on the collection index page in a previous version of Ansible that still has this collection, and use the version selector to navigate to the current version, that you don't see a 404, but instead see the tombstone page.

This unfortunately only makes the tombstone appear in exactly one major version of Ansible - the version where the collection was removed -, but that's better than nothing IMO :)

[felixfontein]

Examples from the Ansible 10 stable docs build:

collections/hpe/index.rst:

.. meta::
  :antsibull-docs: 2.14.0.post0

.. _list_of_collections_hpe:

Collections in the Hpe Namespace
================================

These are the collections with docs hosted on `docs.ansible.com <https://docs.ansible.com/>`_ in the **hpe** namespace.

There is no collection in this namespace.

.. toctree::
    :maxdepth: 1
    :hidden:

collections/hpe/nimble/index.rst:

:orphan:

.. meta::
  :antsibull-docs: 2.14.0.post0


.. _plugins_in_hpe.nimble:

Hpe.Nimble
==========

This collection has been removed from Ansible 10.

If you want to continue using this collection, you can install it manually using
:code:`ansible-galaxy collection install hpe.nimble`.

[felixfontein]

With ansible-community/antsibull-core#174 and ansible-community/ansible-build-data#465 these tombstones are also shown for collections removed from older Ansible major releases.