feat(API): improve get /projects/{project_id}/screenshots/{id} documentation

[Stephen Lumenta (sbl)]

Improves the documentation for get /projects/{project_id}/screenshots/{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 #1168, which was closed after jablan's review.

Feedback addressed from #1168:

Comment	Status
"Internal implementation detail and shouldn't be exposed"	✅ Fixed
"No need to describe id/branch in the description — that belongs in parameters"	✅ Fixed
"429 description is identical for all requests — move to responses.yaml"	✅ Fixed
"It should be possible to hyperlink other endpoints"	⚠️ Not addressed — see note below

What changed:

• branch elaboration is moved to the parameter's own description field in parameters:. The top-level description no longer restates individual request fields.
• Internal detail removed from the description body.
• 429 now uses a bare $ref: "../../responses.yaml#/429" with no inline description override.

Open item — hyperlinks: Related endpoints are referenced in plain prose rather than as Markdown links. If clickable cross-references are a hard requirement, let us know and we'll update the description accordingly.

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