chore(docs,tooling): clean-up old `gh-pages` versions due to large clone size

[danceratopz]

Cloning execution-spec-tests currently involves around 2 Gib of download due to the size of the gh-pages branch, mainly due to the large size of the html docs index file size (search/search_index.json), see #1291 and related PR #1292.

We should:

1. Delete unnecessary versions from the mike version history.
2. Regenerate the index file from retained versions.
3. Delete history (required to reduce clone size!).