[MAINT]: Document naming convention regarding plural and singular

[nickfloyd]

Describe the need

The general pattern has been to use plural on collections and singular on single resource operations.

For instance, repository collaborators (single, collection) implements this approach.

We need to document in our contributor docs this pattern for future work.

SDK Version

No response

API Version

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Relevant log output

Code of Conduct

• I agree to follow this project's Code of Conduct

[ViacheslavKudinov]

This one looks like a duplicate of #2936.

[mkushakov]

Agreed with @ViacheslavKudinov that item 1 duplicates #2936 — that half is in flight in #3234, which adds the singular github_organization_repository_custom_property and deprecates the plural pair.

Item 2 isn't covered by that PR though, and is worth keeping somewhere: the plural-for-collections / singular-for-a-single-object convention isn't written down for contributors anywhere I can find. The closest is .github/instructions/go.instructions.md:66 ("Package names should be singular, not plural"), which is about package names rather than resource and data source names. So this issue is probably best either closed as a duplicate with item 2 split into its own docs issue, or re-scoped to just the docs half.