Deprecate old docs pages

We want to remove references to the legacy client from the public docs. We should do a review of old docs pages, and make sure we have matching samples for the new data client or autogenerated admin client

This may allow us to drop the [patch_devsite_roc.py script](https://github.com/googleapis/python-bigtable/blob/main/docs/scripts/patch_devsite_toc.py), and possibly change devsite renderers (b/345809627)

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As part of this, we need to update any region tags in the [sample folder](https://github.com/googleapis/python-bigtable/tree/main/samples) that currently use the legacy client, to use the autogenerated clients instead. That should update the [public docs](https://cloud.google.com/bigtable/docs/creating-instance#python) to use the new samples

We may also need to re-enable snippetgen for the admin client