Add versioned docs to build-docs task

[clevinson]

Description

Although the Makefile currently has support for building multiple versions of the SDK docs, it sets the path for each version of the site to the corresponding branch name.

This PR enables the version's path prefix to be set independently of the branch name, and adds v0.39 to be compiled from the launchpad/backports branch in addition to master docs.

The default docs site (accessible via the root at docs.cosmos.network) will be the last entry in the ./docs/versions file.

Note: This should not get merged until after #7412 is merged, as launchpad docs currently fail to compile

closes: #6308
ref: #6953

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[fadeev]

Can confirm that this builds two docs websites in separate directories: master and v0.39 👍

[clevinson]

This task fails due to comments not being handled properly in make tasks, fix here: #7420

[lovincyrus]

After merging #7420 → master, I noticed that CI is still failing at the sync-docs step due to the fact it can't locate the aws credentials. Ping-ed @helder-moreira to have a look at this.

I've just merged master → docs-staging as well, it appears to have the same error (unable to locate aws credentials).

[helder-moreira]

@lovincyrus should be fixed in #7444