feat(API): improve post /projects/{project_id}/keys/{id}/key_links documentation

[Stephen Lumenta (sbl)]

Improves the documentation for post /projects/{project_id}/keys/{id}/key_links: 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.

Jira: DEVEX-116

🤖 Generated with Claude Code

[github-actions]

API changelog (oasdiff)

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

2 changes: 0 error, 0 warning, 2 info
info	[api-security-added] at doc/compiled.json
	in API POST /projects/{project_id}/keys/{id}/key_links
		the endpoint scheme security `OAuth2` was added to the API

info	[api-security-added] at doc/compiled.json
	in API POST /projects/{project_id}/keys/{id}/key_links
		the endpoint scheme security `Token` was added to the API