Add unified contributing proposal document (Issue #186)

[vaibhav11123]

Fixes #186 (follow-up proposal)

This PR adds a discussion document in docs/unified-contributing-proposal.md that outlines ideas for improving consistency across contribution workflows in AboutCode projects.

This is not a policy document. It is a proposal meant to start a conversation around common patterns we’ve observed and potential areas where unifying documentation or tools could help contributors.

Happy to revise based on feedback.