[Docs] add response example for bulk entities upload

[spwoodcock]

Issue

• Currently the docs for POST /projects/{projectId}/datasets/{name}/entities show the response type for uploading a single Entity.
• They have not been updated to show the response type when bulk uploading multiple Entities.
• Ideally the response would return the Entity data, but this is not trivial, due to having to re-fetch data from the db for each Entity (line 98 & 113): https://github.com/getodk/central-backend/blob/fb96aa3333d3442ea43365755766262f21de3969/lib/resources/entities.js#L93-L114

Solution

• The endpoint currently returns 'success': true.
• I updated the docs to reflect this, until the endpoint returns the Entity data (if possible).
• I reused the pattern in the docs for return type application/json; extended to use application/json; bulk, allowing us to display two examples side-by-side - not sure if this is the preferred approach!