Add doc-branches for docsite to release CI

[martriay]

Currently the docsite only reflects the contents con main (HEAD). Because of this, old versions are not being showed and therefore old links get broken. For example:

https://docs.openzeppelin.com/contracts-cairo/0.2.1/

would redirect to

https://docs.openzeppelin.com/contracts-cairo/0.3.0/0.2.1/

To fix this, we need to:

• [ ] modify the rule on the docsite to pick up docs-* branches
• [ ] automate the CI to create a docs-vX.Y.0 branch for every major (X) and minor (Y) release
• [ ] manually create branches for existing releases (v0.2.1 and v0.3.0)