Improve rendering for bulk write JSON-like blocks

[isabelatkinson]

json code blocks don't render nicely in github when their contents aren't valid JSON, e.g.:

InsertOne {
   "namespace": "db.coll",
   "document": { "a": <int32> },
}

This PR ports them to javascript blocks, which look better:

InsertOne {
   "namespace": "db.coll",
   "document": { "a": <int32> },
}

Please complete the following before merging:

• [x] Update changelog.
• [ ] Make sure there are generated JSON files from the YAML test files.
• [ ] Test changes in at least one language driver.
• [ ] Test these changes against all server versions and topologies (including standalone, replica set, sharded clusters, and serverless).