Purge rendered doxygen files from the repo

[ehennenfent]

We removed the rendered doxygen files from the repo, but the old ones are still in the history taking up space. We should squash a few commits to get rid of those. Note that there's still much, much older history from SL1 that we don't want to lose, so we can't just squash everything before the commit where we removed the docs.

[woodruffw]

We might want to just postpone this until open sourcing (if we still intend to open source?): we could just create a new genesis commit and then splice the old SL1 history onto it, deleting any artifacts from the history.

[woodruffw]

Subsumed under #381.