pg_docs_bot per michristofides

When you search for Postgres related things, an old version of the documentation is sometimes top of the list (although this was far more of a problem several years ago). Similarly, some links on sites like Stack Overflow are to old versions of the documentation.

pg_docs_bot helps you avoid having to click "Current" when you get there, or the frustration of reading the information before realising it's out of date, or (even worse) not realising at all.

There have been many conversations on the PostgreSQL mailing lists about how to teach search engines which version to link to. In 2021 a huge improvement was made to the docs to let search engines know that the "current" version should be considered canonical, which has helped tremendously.

For the likes of Stack Overflow and blog posts, please do link to the current version whenever you aren't specifically referencing something version-specific.

Features:
* Redirects links to the "current" version
* After redirecting, displays a notice (including a link to the page you were redirected from)
* Avoids redirecting some deprecated features (to avoid 404s)
* Doesn't redirect when coming from another page in the docs, so you can still check old versions (the main problem with generic redirectors)

Limitations:
* Only supports redirecting to "current"
* Works for the English language docs
* Works for version 7.0 links upwards