Add API documentation

[ramonskie]

Summary

• Adds docs/api.md documenting all bosh.io JSON API and download endpoints
• Covers stemcell and release endpoints with request parameters, response schemas, error codes, and curl examples
• Links the new doc from README.md

Endpoints documented

• GET /api/v1/stemcells/{name} — list stemcell versions
• GET /d/stemcells/{name} — redirect to stemcell tarball download
• GET /api/v1/releases/{source} — list release versions
• GET /d/{source} — redirect to release tarball download

[Copilot]

Pull request overview

Adds a new docs/api.md file documenting bosh.io's read-only JSON API and download redirect endpoints for stemcells and releases, and links the new doc from the top-level README.md. This is a documentation-only change; no code or runtime behavior is modified.

Changes:

• New docs/api.md covering four endpoints with path/query parameters, response schemas, error codes, and curl examples.
• README.md gains a bullet linking to the new API doc.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/api.md	New reference documentation for /api/v1/stemcells/{name}, /d/stemcells/{name}, /api/v1/releases/{source}, and /d/{source}.
README.md	Adds a link to the new API doc.

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