Thread guide

[AbstractUmbra]

Summary

This PR adds a section on the guide for threads.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[scarletcafe]

I already reviewed and revised this one so I think it's good, RFC from others I guess.

[Rapptz]

This is mostly fine, but can we add a section about how to deal with threads when you receive a message from them.

[Soheab]

This needs a section or should mention something about forum channels, "posts" and tags (get, create, add).

[AbstractUmbra]

This is mostly fine, but can we add a section about how to deal with threads when you receive a message from them.

Started on this but needs more fleshing out. Having a bit of issues trying to think of gotchas relating to this to add in. Suggestions on this are very welcome!

[tailoric]

• One inaccuracy about public threads in the guide.
• Otherwise some nitpicks and corrections to the wording.
• Two small wishes to expand on some examples.

Overall I'd say however the guide is pretty far and well written and should give a complete overview over the capabilities of threads (unless I missed something)

[Soheab]

Just two small things, looks good and has everything relevant otherwise.

[tailoric]

I don't see anything to fix or add right now so
LGTM

[No767]

I will review this sometime this week for my input

[No767]

Left my thoughts below