Define two new subteams for rustdoc to better split everyone's work

[GuillaumeGomez]

Follow-up of rust-lang/rust-forge#993.

As discussed in the rust-forge, to better reflect what everyone is working on and to ensure people are only pinged to FCPs relative to what they're interested into, this PR creates two new subteams.

Important: If a member is not in any subteam, it means they will only be pinged on general FCPs.

Important 2: Please don't hesitate to tell me if someone wants to be added/removed from one of the groups so I can update the PR before it's merged.

cc @rust-lang/rustdoc

[jieyouxu]

Waiting a bit for rustdoc team members in case they have any requested changes.

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Team Diffs:
    ➕ Creating team:
      Org: rust-lang
      Name: rustdoc-internals
      Description: Managed by the rust-lang/team repository.
      Privacy: closed
      Members:
        GuillaumeGomez: member
        Manishearth: member
        Urgau: member
        camelid: member
        fmease: member
        lolbinarycat: member
        notriddle: member
        yotamofek: member
    ➕ Creating team:
      Org: rust-lang-nursery
      Name: rustdoc-internals
      Description: Managed by the rust-lang/team repository.
      Privacy: closed
      Members:
        GuillaumeGomez: member
        Manishearth: member
        Urgau: member
        camelid: member
        fmease: member
        lolbinarycat: member
        notriddle: member
        yotamofek: member
    ➕ Creating team:
      Org: rust-lang
      Name: rustdoc-json-backend
      Description: Managed by the rust-lang/team repository.
      Privacy: closed
      Members:
        GuillaumeGomez: member
        Urgau: member
        aDotInTheVoid: member
    ➕ Creating team:
      Org: rust-lang-nursery
      Name: rustdoc-json-backend
      Description: Managed by the rust-lang/team repository.
      Privacy: closed
      Members:
        GuillaumeGomez: member
        Urgau: member
        aDotInTheVoid: member

[Manishearth]

I don't mind being on internals!

[GuillaumeGomez]

Added you to internals then. :)

[Urgau]

I have not being much active in recent months with the json backend, but I wouldn't mind being on the json team.

[GuillaumeGomez]

Done as well.

[jieyouxu]

(Merging for now, further changes can be in follow-ups.)