Add docs stub for recovery.conf

Hi all

I noticed that when recovery.conf was removed in 2dedf4d9a8 (yay!) the docs
for it were removed completely as well. That's largely sensible, but is
confusing when users have upgraded and are trying to find out what
happened, or how to configure equivalent functionality.

https://www.postgresql.org/docs/11/recovery-config.html just vanished for
/12/, and as a result
https://www.postgresql.org/docs/current/recovery-config.html is a 404. I
think that's unhelpful since we encourage people to use /current/ links.

The attached patch restores the recovery-config.html page with a short note
explaining why it's gone and what to do instead. It's added to a new
appendix "Obsolete or renamed features, settings and files".

I found it remarkably hard to find out what exactly made a "standby server"
actually be a standby server in the docs so I have added a couple of
cross-references that make the role of the standby.signal file much more
discoverable from relevant locations.

I propose a policy that we preserve our <chapter> and <sect1> ids. We
should move them to an "obsolete" section at the end, like the one I
created here, and provide stubs for them instead of removing them. That'll
help prevent us from breaking links on the wider web, in 3rd party
documentation, etc.