Automated issue triage and guidance for community PRs

## Description

An overhaul of the current issue triage system in aws-cdk.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @conroyka
| Author(s) | @conroyka
| API Bar Raiser | @mrgrain
| Stakeholders |

> See [RFC Process](https://github.com/aws/aws-cdk-rfcs#rfc-process) for details

## Workflow

- [x] Tracking issue created (label: `status/proposed`)
- [x] API bar raiser assigned (ping us at [#aws-cdk-rfcs](https://cdk-dev.slack.com/archives/C025ZFGMUCD) if needed)
- [x] Kick off meeting
- [x] RFC pull request submitted (label: `status/review`)
- [x] Community reach out (via Slack and/or Twitter)
- [ ] API signed-off (label `status/api-approved` applied to pull request)
- [ ] Final comments period (label: `status/final-comments-period`)
- [ ] Approved and merged (label: `status/approved`)
- [ ] Execution plan submitted (label: `status/planning`)
- [ ] Plan approved and merged (label: `status/implementing`)
- [ ] Implementation complete (label: `status/done`)

---

> Author is responsible to progress the RFC according to this checklist, and
apply the relevant labels to this issue so that the RFC table in README gets
updated.

Start with the linked RFC Process and review the completed and unchecked items in the issue's workflow. No implementation files, tests, or entry points are named; the issue's stated completion point is implementation complete, after API approval, final comments, approval, and an execution plan.

Written by the indexing model from the issue text.