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

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 be replaced with a simple --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.

Inizia con le pagine della documentazione command_line.html e error_code_list2.html, seguendo i link esistenti per flag come --warn-unreachable e codici di errore come unreachable. Confronta i flag CLI interessati, i nomi delle configurazioni, i codici di errore e le sezioni della documentazione correlate, quindi assicurati che i riferimenti pertinenti funzionino in entrambe le direzioni.