Remove old versions from the doc-site

[ericnordelo]

🧐 Motivation
We have a lot of versions shown in the dropdown, we can start removing at least the versions previous to 0.7.0 since that's cairo 0 which we won't maintain anymore.

📝 Details

After merging this PR in docs, we can achieve this by removing the old release branches (note that this doesn't mean removing the the release tags).

[ericnordelo]

I have removed every release branch previous to v0.7.0.

[ericnordelo]

Closing this as it should be automatically fixed when we start using the doc branches.