Use rustbot config to configure rendered links

[Urgau]

This PR adds the new config related to the rendered links.

cc rust-lang/triagebot#1868
cc @ehuss

[rustbot]

Error: Invalid triagebot.toml at position 3:2:

TOML parse error at line 3, column 2
  |
3 | [rendered-link]
  |  ^^^^^^^^^^^^^
unknown field `rendered-link`, expected one of `relabel`, `assign`, `ping`, `nominate`, `prioritize`, `major-change`, `glacier`, `close`, `autolabel`, `notify-zulip`, `github-releases`, `review-submitted`, `review-requested`, `shortcut`, `note`, `mentions`, `no-merges`, `validate-config`, `pr-tracking`, `transfer`, `merge-conflicts`, `bot-pull-requests`

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.