Documentation Improvements

[ApCoder123]

I've made a couple of modifications to the documentation:

• Consistency (Phrase "two factor authentication", rather than a mixture of "two factor auth" and "Two Factor Authentication")
• Remove outdated references (there were a couple of references to the old YAML schema in CONTRIBUTING.md)
• Move new categories to end of contributing guidelines (the majority of contributors are looking to add sites)
• A couple of minor language changes & typo fixes
• Mark v2 API as deprecated
• Remove Gitter link (it doesn't seem to have been used properly since 2019)