Various improvements to CONTRIBUTING.md

[AetherUnbound]

Description

While looking at our CONTRIBUTING.md file, I noticed a number of improvements that could be made:

1. We should link to the CONTRIBUTING.md file in our main repo, as it has a lot of depth to it: https://github.com/WordPress/openverse/blob/0b5e24bd325da7b75db9484a268d7c8d197ae920/CONTRIBUTING.md
2. We should remove the reference to CODEBASE.md, which no longer exists: https://github.com/WordPress/openverse-api/blob/main/CONTRIBUTING.md?plain=1#L11
3. We should add a reference to DOCUMENTATION_GUIDELINES.md: https://github.com/WordPress/openverse-api/blob/429fd45916c9e064ccea772afc184466304bce4e/DOCUMENTATION_GUIDELINES.md
4. We should remove the line about GitHub discussions (they are currently disabled) in both places its referenced: https://github.com/WordPress/openverse-api/blob/main/CONTRIBUTING.md?plain=1#L21 https://github.com/WordPress/openverse-api/blob/main/CONTRIBUTING.md?plain=1#L97

Additional context

Inspired by https://github.com/WordPress/openverse-catalog/issues/775

Resolution

• [ ] 🙋 I would be interested in resolving this bug.