chore: initialize archival of version 0.25

[pepopowitz]

Note that this PR is Very Large due to removing all other versions from the site. It will crush your browser, and I'm sorry!

Description

Part of #1173.

Begins the archival process for version 0.25, by isolating the version in its own site.

This PR will be merged into the unsupported/0.25 branch! It will never make it to main.

The steps completed in this PR are:

1. Remove other versions from the version manifest file
2. Delete source for all other versions, including sidebars
3. Add a robots.txt file to prevent crawling of the extracted site
4. Update the Dockerfile to allow for local debugging of the extracted site

This isn't comprehensive for the isolated site, but it begins the process. The remaining isolation work for v0.25 will follow in future PRs.

I screwed up with the unsupported/0.25 branch!

As of right now, the unsupported/0.25 branch points at the commit where the last changes occurred in the 0.25 docs. This is wrong! I should have based it on #2155, because I need that pre-work! I'm working on fixing/rebasing, but if you look at this and it looks like a step back in time, that's why (and I'm fixing it!)

I will likely merge this before it is green

The build will fail until I get just about all of the steps done for isolating. It's not making it to main, so if it is merged while red, it won't affect anyone else, but it will give me the chance to break the steps down into smaller PRs.

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant directory, or they are not for an already released version.
• I have added changes to the main directory (aka ), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[pepopowitz]

Closing in favor of #2165. As I mentioned in the body of this PR, I messed up the unsupported/0.25 branch by pointing it to an old commit. Rather than trying to preserve things in this PR after a rebasing adventure, I just created a new one.