Add ability to create a media-only forum channel

[Soheab]

Summary

This PR makes it possible to create a media-only forum channel by setting media to True in Guild.create_forum().

Not tested as IIRC it requires some sort of subscription.

Checklist

• [ ] 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, ...)

[LuciBryden]

For context on Media Channels:

1. Server Shop - U.S. discord users can enable Monetization on their server - which grants access to creating Media Channels
2. Media Channels

[Soheab]

Run black.

Done.