feat(API): improve delete /projects/{project_id}/documents/{id} documentation

[Stephen Lumenta (sbl)]

Improves the documentation for delete /projects/{project_id}/documents/{id}: sharper descriptions, parameter docs, error responses, and usage examples.

Drafted with AI assistance and grounded in the API implementation. Please review for technical accuracy before merging; nothing is merged automatically.

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[github-actions]

API changelog (oasdiff)

Doc-only edits (descriptions, examples) do not appear here.

No changes to report, but the specs are different.
Run 'oasdiff diff' to see structural differences.

[Stephen Lumenta (sbl)]

This PR replaces #1177, which was closed after jablan's review.

Feedback addressed from #1177:

Comment	Status
"numeric primary key is an internal development detail and should not be exposed"	✅ Fixed
"This 403 description is almost fully generic and could be moved to the referenced section"	✅ Fixed

What changed:

• The description no longer mentions numeric primary key or DocumentPolicy#destroy?. Implementation details are translated into caller-facing behaviour: "Requires the write scope on the access token and manage permission on the document."
• The 403 description is now endpoint-specific: "Obtain a token with the write scope or request manage access from a project administrator." Generic 403 prose has been removed rather than moved inline.

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