ci: publish versioned urls for docs

[conorsch]

Our docs always live under main in the URL route for the guide: https://guide.penumbra.zone/main/pd/build.html However, we don't update those docs in line with releases, so the information on main is not correct for testnet users. See recent changes in #3395, for instance: the docs were updated to remove mention of --home, but that's only true of the main branch, not of the currently running v0.63.2 release.

We should continue to update docs on merge into main, but also tag new versions during releases, e.g. https://guide.penumbra.zone/v0.63.2/pd/build.html. That will help us communicating about docs.

[hdevalence]

xref #6

[conorsch]

I still want this problem solved. From the historical issue, looks like Firebase can't support this use case as described. Any objections to simplifying the URLs in the meantime? I'd rather see https://guide.penumbra.zone/ in my browser than https://guide.penumbra.zone/main/index.html, especially if I cannot navigate to https://guide.penumbra.zone/v0.71.0/index.html. We can specify redirects in the Firebase config so that historical URLs out in the wild don't break for people—that's what led to #4232 anyway.

It's no problem to migrate to historical tags later on: we can always check out versions and export the html and stick it in a subdir if we care about doing that.