Update lang-docs membership

[ehuss]

This updates the lang-docs membership to better reflect the current state. TC has been helping with the organizational side of lang-docs, and helping across all of the projects.

I'd like to express a huge appreciation to Gankra, Havvy, and matthewjasper for their contributions, and they are always welcome to come back.

cc @rust-lang/lang-docs

[github-actions]

Dry-run check results

[WARN  sync_team] sync-team is running in dry mode, no changes will be applied.
[INFO  sync_team] synchronizing github
[INFO  sync_team] 💻 Team Diffs:
    📝 Editing team 'rust-lang/all':
      Deleting member 'Gankra'
      Deleting member 'Havvy'
    📝 Editing team 'rust-lang/inside-rust-reviewers':
      Deleting member 'JohnTitor'
    📝 Editing team 'rust-lang/lang-docs':
      Deleting member 'Havvy'
      Deleting member 'Gankra'
      Deleting member 'matthewjasper'
    📝 Editing team 'rust-lang-nursery/lang-docs':
      Deleting member 'Gankra'
      Deleting member 'matthewjasper'
    📝 Editing team 'rust-lang/leads':
      Deleting member 'JohnTitor'

[traviscross]

As @ehuss said, huge thanks to Gankra, Havvy, and matthewjasper.

This is OK to merge.

cc @rust-lang/team-repo-admins