[Documentation] FAQ, Contributing Guidelines, Wiki

[JacksonBurns]

There are a few places in the RMG documentation that need to be updated:

• Our FAQ is quite outdated, offering advice ranging from unhelpful to wrong - we need to give it a once-over and make sure everything is UTD. See: https://reactionmechanismgenerator.github.io/RMG-Py/users/rmg/faq.html#installing-rmg (e.g. the Installing RMG section offers suggestions about Windows Binary installs, which we no longer support).
• The Contributing guidelines in the wiki are hidden away from possible contributors and out of date - they should be modernized and a link to them should be added to the pull request template.
• The contents of the Wiki should be moved to the documentation directory where still relevant, and deleted otherwise.

@jonwzheng and I are working on this!

[jonwzheng]

Current plan is to move contributor guidelines to a CONTRIBUTE.md file, as described here.

[jonwzheng]

edit: will track wiki update progress in main comment of PR #2703