[docs]: Create Deprecation page per deprecation group

[niklas-wortmann]

This is a rough POC to see if it works for our purpose to improve the deprecation messages.

Text might need some improvement/additional information
@benlesh @cartant

[cartant]

Fixed some typos that were pointed out above and rebased onto master.

There are some deprecation messages that still include the {@ ... } markup that's discussed here. IMO, it would be best to remove these ATM, as they clutter the output in VS code and on the command line. Apart from that, this LGTM and, IMO, improvements regarding the deprecations need to be incorporated into the docs ASAP. I suppose it's too late/difficult to make changes to v6, at this stage, but v7 needs better messages and docs before it can be released, IMO.

[cartant]

FWIW, we really should try to get the deprecation messages sorted before this lands:

https://mobile.twitter.com/WenluWang/status/1275458404132073473

We should really fix them in v6 too, as people are going to be using that for a while in Angular, etc.