docs: Good first issues & bounties page (addresses #2481)

[Bennethxyz]

Adds a docs page that consolidates links to good-first-issue and bounty queries, includes a concise summary of bounty rules, and links back to development/testing/style-guide. Also links it from the docs index and CONTRIBUTING.md.\n\nThis implements the call in #2481 to surface good-first-issue and bounty guidance for newcomers.