Add API doc for CI server endpoints

[mdinler]

There are three endpoints:

• POST /push-events
• GET /history
• GET /history/{id}

[mdinler]

Please describe the status codes returned and the response body for each endpoint.

[spelgubbe]

POST /push-events:

204 (no content) always.

GET /history:

200 (ok) always. (if history is empty then it returns an empty array)

GET /history/{id}:

400 (bad request) if id is not a number
404 (not found) if build id doesn't exist
else 200 (ok)