Add git blame ignore

[freddyheppell]

Adds git blame ignore to prevent most code in the repo being attributed to #2033.

GitHub will pick up on this by default.

Using Git locally, you can use this by passing the --ignore-revs-file argument e.g.

$ git blame bertopic/_bertopic.py --ignore-revs-file .git-blame-ignore-revs

or run this config to use by default

$ git config blame.ignoreRevsFile .git-blame-ignore-revs

[MaartenGr]

Awesome, thanks for the PR!