Add a redirect to the v2 docs to the v1 gh-pages site

[tbarsballe]

Recently, we had some users landing on the old v1 docs rather than the v2 docs. As we do still have some users on v1, I think it is important to keep those docs around, but they should have a link to v2 so that new users don't get lost. This PR adds a deprecation notice and link to the top of the v1 docs:

The old v1 docs appear to be entirely static rather than the more typical Jekyll, so this change is done in JS, so it applies to all pages with minimal effort and duplication. Not optimal, but for a deprecated doc site that isn't going to change anytime soon I think its fine.

Proposed Changes:

For inclusion in changelog (if applicable):

1. Update the v1 gh-pages documentation with a deprecation notice and a link to the v2 documentation

PR Checklist:

• [x] This PR is as small and focused as possible
• [x] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• [x] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• [x] This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications:

[ischneider]

Nice, thanks!

I suppose we can check SEO after but my initial thought was a custom sphinx header or such (so JS is not required), though I certainly appreciate the simplicity of this approach...

[tbarsballe]

@ischneider Yeah, that'd definitely be the cleaner solution. I poked around the repo and frankly didn't want to try untangling the 4+ year-old doc build situation, this was the simplest and fastest solution I could throw together to solve this while still looking half-decent.