Search: allow deleting search index for a project (#12584)
This allows us to mark a project so it's never indexed for search.
Currently, we will mark these projects manually and create the
notification so users are aware of this change and can re-enable search
indexing in case we made a mistake without contacting support.
ref
https://github.com/readthedocs/meta/issues/197#issuecomment-3518456511
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>