docs: fixes of PVP-150-1 [2.X]

[michalChrobot]

This PR focuses on fixing errors related to PVP-150-1 "XmlDoc should be valid"

At the beginning I wanted to fix both PVP-150-1 as well as PVP-151-1 in one PR but I came to the conclusion that it's better to separate those because of the amount of changes to be checked for correctness.

There were several places (like for example FastBufferWriter/FastBufferReader) where XmlDoc was describing unused parameter <param name="unused">An unused parameter used for enabling overload resolution based on generic constraints</param>, we should ensure that it's correct that said argument is not used by the given APIs.

Additionally pvpException file was moved out of the package and package-pack and package-test were corrected with proper usage of upm-pvp and proper exceptions checking

[NoelStephensUnity]

I think we are still missing some XML documentation, but this is a very good start!
🥇

[michalChrobot]

I did it based on PVP exceptions I saw in the file, it may happen that some new places were introduced since I created this branch. That's why I would like to merge CI changes (LINK) first since this will detect (and block) any new "introduction of related errors" so we can make sure we are addressing all XmlDoc issues (this is different from API docs and there will be a separate PR for those (LINK), it's still work in progress though, I may have it next week)