docs(rome_diagnostics): add a contributor documentation file for diagnostics

[leops]

Summary

This PR adds a new file in crates/rome_diagnostics/CONTRIBUTING.md that serves as a documentation on how to write diagnostics, with both technical explanations and best practices to write good errors for the user.

Test Plan

N/A

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[github-actions]

Playground for commit 2ba6943

[ematipico]

Here's some feedback after working with the new diagnostics:

• it wasn't clear that diagnostics can have their source code and file injected via some external API (.with_file_path and with_file_source_code), if these diagnostics need to show some code frame
• if the diagnostic need to show an error as code frame, they need to a span assigned to them
• it would be nice to have an utility to print diagnostics in tests (we had an Emitter, we could have it for these new diagnostics too)
• we should explain when a diagnostic can be converted to Error. I managed to pick it up, but we should explain some use case

[ematipico]

@leops should we start merging this PR and then continue with other PRs? There haven't been updates recently

[leops]

@leops should we start merging this PR and then continue with other PRs? There haven't been updates recently

Ah yes I wanted to add an additional section that show what a diagnostic looks like when it gets printed along with all the advice types, but we could merge this now and extend it later