Docs: Add partner docs directory; Add module management doc

[ebinnion]

Our partners have occasionally requested the ability to manage modules. We currently support this functionality via a couple of filters, but it's not widely known. Creating this document in the docs/partners directory will make the information more discoverable for partners.

As a note, while creating this documentation, it became apparent that while we support forcing a module on/off, we do not properly handle this in the UI. I've created #8800 to address this.

We intend on adding more partner documentation into this docs/partners directory in the future. This will allow partners to easily access the documentation as well as view updates without us having to manually export a PDF and send it to the partners.

cc @gravityrail @jessefriedman @griersongithub @nicoleckohler

changelog entery

• We improved the documentation for partners so they're now able to get them right on the Jetpack's Github repository under the docs/partners directory.