Rewrite the SyntaxExtension/Hygiene chapter

[Veykril]

It's really not great atm but I'm currently having trouble going into the topic without touching macro_rules!/proc-macro specifics, https://veykril.github.io/tlborm/syntax-extensions/hygiene.html

Basically after #43 lands, the hygiene subchapter of the syntax extension chapter is still a lot worse than the declarative macros one. Feels like a lot of information from the declarative one should be lifted into the syntax extension one?

[Hasnabruzzn]

Just opened #78 on the duplicated evaluation problem, which might be part of the hygiene chapter.
What is so bad about the chapter?
What I got from it is:

• it's bad to rely within the macro on things defined outside the macro (like assuming a specific variable is there)
• it's bad to change the things outside the of the scope of the macro (like creating variables)
• Rust allows you to do those bad things, so you are responsible.