Docs: 3.7 deprecations and revisions

Several docs tweaks and updates related to deprecations and changes in the 3.7 release. When this PR is merged, please resolve the following tickets: 

- [DOC-1129](https://tickets.puppetlabs.com/browse/DOC-1129)
- [DOC-1131](https://tickets.puppetlabs.com/browse/DOC-1131)
- [DOC-1143](https://tickets.puppetlabs.com/browse/DOC-1143)
- [DOC-1147](https://tickets.puppetlabs.com/browse/DOC-1147)
- [PUP-796](https://tickets.puppetlabs.com/browse/PUP-796)