[Docs] Drop versioned docs

[markerikson]

React-Redux currently uses Docusaurus's "versioned" docs feature. We show docs for versions 5.x, 6.x, 7.0, 7.1, and 7.2

However, this setup is very confusing for both external contributors and us as maintainers. Due to how the versioning works, the /docs folder in the repo actually represents the "Next" version, which doesn't even show in the published docs by default - the 7.2 version is "Current". Contributors typically make edits to files in /docs, and we have to keep telling them "oh, also apply the same edits to the duplicated files in the /website/versioned_docs/ folders as well".

Our API has been mostly stable over time - there's just a few changes to the connect arguments between v5, v6, and v7.

Looking at the last year's worth of traffic:

• /api/hooks: 840K hits
• / : 600K (less than the hooks page)
• /6.x/quick-start: 6K
• /5.x/api/connect: 7K

I'd rather have one copy of the docs that just says "added in 7.2", etc.

After discussing with Tim, it seems like we should just drop the versioned docs aspect entirely.

This will require:

• Figuring out how to reconfigure Docusaurus to turn off versioned docs
• Updating any relevant API reference sections to indicate when a particular option was added or changed
• Removing those versioned_docs files
• Adding redirects to point any now-dead URLs like /6.x/ to the relevant current page instead.

[markerikson]

Done! I've nuked the versioning setup and redirected all the old version sections to point to the current equivalents.

I haven't gone through and tried to do a line-by-line check on the API references to figure out when specific options were added. I know forwardRef was added in v7.0. Don't think anyone's gonna worry about that too much at this point, though.