Implement documentation versioning

[Yantrio]

Docusaurus supports documentation versioning out of the box, we should investigate the best approach for us to have here.

Initial thoughts are as follows:

• Main branch of docs should be the "next" build (or preview, etc)
• Version each set of documentation as we do a release, this should be automated in the long run.
• Older versions of opentf docs should be kept where possible, so this effort needs to be introduced earlier rather than later.

[Yantrio]

For now, we can use the https://docusaurus.io/docs/versioning setup manually.

However, I would like the documentation to be stable before we start implementing versioning.

My proposal for a versioning approach would be the 2nd approach listed here: https://docusaurus.io/docs/versioning#configuring-versioning-behavior

Tl;dr

• Next version is the live docs from the main repo, this goes unpublished
• Current version is the latest version that is released
• Older versions are maintained and created with the cli tool when we do a release process

IMHO, it is not worth automating this right now as the process will happen so irregularly that we can handle it manually for now.

What we will need to think about in the future is how we can allow people to contribute to older versions of documentation and make sure that this is always an option (ie, to fix language spec docs)

[ghost]

Closing in favor of #257 which already has an assignee.