Add a getting started section for new contributors

[mikemhenry]

We have had a wonderful influx of people who want to contribute to signac. Many have the same question, "where do I start?" I think we should add a section here that links to our github issues with the "good first issue" tag:

https://docs.signac.io/en/latest/community.html#contributions

Any other ideas?

[mikemhenry]

Example link: https://github.com/glotzerlab/signac-flow/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22

[vyasr]

I think a few of these links would be helpful. It would be helpful to link separately to signac and signac-flow, and maybe to include a separate list by label (especially the docs label for people who would feel most comfortable starting with docs).