docs: added a page on dongles

[Nick-Munnich]

Added a page on configuring dongles, building on the work that @RasmusKoit did in #2401. The method recommended is personal to each user, but works for both boards and shield based keyboards (requires a board for the dongle, to avoid users messing with flash etc).

Subsumes #2401, closes #2044.

EDIT: I am waiting until #2537 to make some changes.

[caksoylar]

It might be good to add a pointer from split keyboards feature page to here, saying it is also used for creating "dongles".

I think this is well written overall. As a side note, it is a bit too prescriptive for my taste (do this, then do this, rather than explaining how the config works) but given the current complexity of the setup it seems unavoidable, assuming we do want to document how to do it.

[Nick-Munnich]

As a side note, it is a bit too prescriptive for my taste (do this, then do this, rather than explaining how the config works) but given the current complexity of the setup it seems unavoidable, assuming we do want to document how to do it.

This was intentional, since we keep getting people asking for help setting up a dongle for their keyboard who have not done the whole shield porting thing. I see the dongle setup as just a highly simplified and prescriptive version of the shield docs. I'll adjust to refer readers to the shield guide a bit stronger if they want to understand any details.