Support Guild Onboarding

[bijij]

Summary

This pull request implements support for the guild onboarding configuration.

Checklist

• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] 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, ...)

[codeofandrin]

A few fields are missing, which I get:

"enable_default_channels": true,
"enable_onboarding_prompts": true,
"onboarding_prompts_seen": {},
"onboarding_responses_seen": {},
"responses": []

This would belong to Onboarding

[codeofandrin]

The guild features could be added to this PR too:

• GUILD_ONBOARDING_EVER_ENABLED
• GUILD_SERVER_GUIDE
• GUILD_ONBOARDING
• GUILD_ONBOARDING_HAS_PROMPTS

Also the audit log events: https://github.com/discord/discord-api-docs/pull/6041

[codeofandrin]

The fields of the prompt option have changed: https://github.com/discord/discord-api-docs/pull/6479

Looks like the emoji isn't implemented at all for editing yet.

[codeofandrin]

The name of some audit log actions have changed and 2 new actions have been added, see https://github.com/discord/discord-api-docs/pull/6041

I think it doesn't make sense to change the name of the server guide actions too, because the description kept the same and so the current names are more meaningful.