MEME-2032 Updated Template Engine API with current template and version response format

This is for Jira issue MEME-2032, removing the default_version_id field from template operations. I did some additional cleanup to ensure that the documentation reflects the current API.

https://jira.sendgrid.net/browse/MEME-2032

Note that these changes reflect the current API in production and do not reflect the Template Engine changes documented separately in https://github.com/sendgrid/docs/pull/431 (that change is not yet live).