Improve the `Contribution workflow` explaining how to determine what is available for external contributors

I’d like to contribute further, but it is difficult to identify suitable issues. The [`help wanted`](https://github.com/dependabot/dependabot-core/issues?q=is%3Aopen+is%3Aissue+label%3Ahelp-wanted) label currently has only two issues, while [`good first issue`](https://github.com/dependabot/dependabot-core/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) has 16 out of 726 open issues.

Could the [Contribution workflow](https://github.com/dependabot/dependabot-core/blob/main/CONTRIBUTING.md#contribution-workflow) be expanded to clarify which bugs and feature requests are open to community contributions?

One challenge I have noticed is that **issues are often assigned to team members quickly**. This can create the impression that they are not open to external contributions, even when that is not necessarily the case. As the backlog grows and the team’s capacity remains finite, external contributions may ultimately be the most efficient way to move some of these issues forward.

For other issues, it is difficult to know whether the core team supports the proposed change, is already working on it, or would review and accept a PR.

I understand that maintaining labels and contributor guidance takes effort, but greater clarity would help contributors identify useful work and avoid investing time in changes that may not be accepted.