api/doc: add manifest schema

ported the manifest from https://github.com/GoogleCloudPlatform/container-agent

a few notes:
- it's using draft v4
- I reused the description from https://developers.google.com/compute/docs/containers/container_vms#container_manifest
- I tested it against the simple `pod.json` example
```
>>> import json
>>> pod = json.loads(open('pod.json').read())
>>> manifest = pod['desiredState']['manifest']
>>> import jsonschema
>>> schema = json.loads(open('../doc/manifest-schema.json').read())
>>> jsonschema.validate(manifest, schema)
>>> 
```
- I didn't link it yet in the documentation (I have yet to figure out how to reference a subresource schema from an external file)

/cc @bgrant0607