chore(ci): truncate gh-pages branch history

[hsheth2]

This should significantly reduce the size of our git repo. My informal checks indicated that the gh-pages branch was responsible for over 50% of our repo size.

Change suggested by https://www.reddit.com/r/rust/comments/wy3j50/psa_if_youre_using_ghpages_to_host_your/

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub