[Docs] Update contributing guide with governance automation rules

## Summary

The contributing guide at `src/contributing.md` needs to be updated to document the new contributor governance automation being rolled out via [Kuadrant/.github#3](https://github.com/Kuadrant/.github/pull/3).

## What needs to change

The following sections should be added or updated to reflect the automated governance workflow:

### Issue triage labels

Document the new automatic triage labeling:

| Label | Meaning |
|---|---|
| `triage/needs-triage` | Auto-applied to new issues, awaiting maintainer review |
| `triage/accepted` | Reviewed, prioritised, and ready for work |

Only maintainers can change triage labels.

### PR requirements for external contributors

Every PR from an external contributor must:

1. **Link to an issue** — use `Fixes #123` or `Closes #123` in the PR description, or link via the GitHub sidebar
2. **Link to a triaged issue** — the linked issue must have the `triage/accepted` label
3. **One PR at a time per issue** — the linked issue must not already have an open PR from another contributor

PRs that don't meet these requirements will be **automatically closed**. The branch won't be deleted, so contributors can reopen once requirements are met.

### Existing sections to review

- **"Find an Issue"** — could reference `triage/accepted` as the signal that an issue is ready to work on
- **"Submitting Pull Requests"** — the existing guidance about referencing issues should be strengthened now that it's enforced by automation
- **"Assigning an Issue"** — one-PR-per-issue is now enforced, not just encouraged

## How to review

- Compare the existing `src/contributing.md` with the `CONTRIBUTING.md` added in [Kuadrant/.github#3](https://github.com/Kuadrant/.github/pull/3)
- Ensure the docs site version covers everything the org-level CONTRIBUTING.md covers, with friendlier prose

## Additional context

- Governance workflow: [`Kuadrant/.github#3`](https://github.com/Kuadrant/.github/pull/3)
- Test repo: [`Kuadrant/governance-test`](https://github.com/Kuadrant/governance-test)