Redirect to new documentation pages

[sbliven]

This repo has been replaced by SciCatProject/user-documentation.

1. Update README with a callout warning developers not to use this repo, plus a link to the current contributor guide for documentation
2. Add a callout to all pages with a deprecation warning
3. Replace all content pages with redirects to user-documentation, if available
4. After 6 months, disable serving with Github Pages
5. Archive the repo