[DOCS] Custom Salt Extension Installation

[NoRePercussions]

Description
I can only find clear documentation about how to install community salt extension published to pypi, and not an unpublished internal extension.

Additionally, I cannot find documentation about how to use a git target for salt modules/extensions, only local installation.

This makes extensions difficult to adopt in the same way formulas are.

Type of documentation
Salt documentation, the salt extensions page, add keywords to the salt master configuration page.