docs-rs dev-guide: deploy to gh pages

[syphar]

template used: https://github.com/rust-lang/team/blob/main/repos/rust-lang/async-book.toml
docs.rs part: rust-lang/docs.rs#3456

[syphar]

r? @ubiratansoares @marcoieni

[rustbot]

Error: The feature assign is not enabled in this repository.
To enable it add its section in the triagebot.toml in the root of the repository.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

[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] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/docs.rs':
      Environments:
        ➕ Create: github-pages
            Branches: main
      Pages:
        ➕ Create: workflow