refactor: update userdev docs to improve clarity

[Leguan16]

As written In the discord channel

I feel like the paperweight section needs a bit of cleanup and rewording.

I don’t know if the „1.20.5 and beyond“ still make sense since reobf/spigot mappings isn’t a thing anymore. It kinda confused me until i saw the section above it. IMO we should either move the new part below the old part so the „and beyond“ still makes sense, or we reword that old part entirely.

This PR just adjust headers so the 1.20.5 section is nested into the reobfuscation section which makes more sense. I opted out of trying to reword it but i am open to do that as well if the consensus is to do so.