meta: Notify Zulip for rustdoc nominated issues

meta: Notify Zulip for rustdoc nominated issues

The rustdoc team does not currently use the `I-nominated` label, unlike
the libs and compiler teams (and maybe others). One reason for this is
that the other teams discuss their nominated issues in meetings, while
rustdoc is an async-only team.

However, it might be helpful to start using the `I-nominated` label for
rustdoc. The team currently uses a `cc @rust-lang/rustdoc` ping as the
equivalent, but it's easier to track issues when they use `I-nominated`.
Also we'd be more consistent with the other teams' procedures.

Since rustdoc doesn't have meetings, I propose we instead use the
triagebot notify Zulip functionality to create a topic in `#rustdoc` on
Zulip and ping the team. So it would look a bit like the procedure for
WG-prioritization when an issue acquires the `I-prioritize` label.