Complete bi-directional linking between cli-flags/configs and error codes

[wyattscarpenter]

Could there also be links (both ways) between those and the documentation of the different error codes that are affected by the configuration options (eg. between --disallow-any-generics, disallow_any_generics and type-arg). In this example, there is also the "Generics" section of the documentation.

Some of those links are already there, some are not there yet.

Originally posted by @Zeckie in #12347

I agree with Zeckie, having just spent some time looking at --warn-unreachable and thinking "hmm, wasn't this some kind of code as well?" and then remembering [unreachable]

Possibly, flags which could just be --enable-error-code whatever should be deprecated in some way. (Although, it probably doesn't make sense to remove them, since they're so cheap to maintain.) But that would be a different issue, anyway.

This bi-directional linking could probably be achieved for free an automatically if more of the docs were automatically generated, a perennial thought of mine.

If the other issue merging config and cli guides is fixed, then this issue would naturally become easier, or perhaps get fixed along the way.