[commands] Update docs to include proper walkthrough

[IAmTomahawkx]

Pull request summary

This PR creates proper documentation for the commands ext, including a proper walkthrough and explanation of creating commands and parser capabilities.

Additionally, this fixes a bug where the PartialChatter converter depended on the chatter cache.

This PR requires #425

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
  • [ ] I have updated the changelog with a quick recap of my changes.
• [x] 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)
• [x] This PR contains non-code changes (e.g. documentation, README, ...)

[spofdamon]

This is really helpful, thanks.

[chillymosh]

LGTM other than would reformat /home/runner/work/TwitchIO/TwitchIO/twitchio/ext/eventsub/models.py