[FEATURE] Documentation for uORB gnss_lowerhalf

[linguini1]

Is your feature request related to a problem? Please describe.

There is a generic lower-half driver for GNSS modules which is currently undocumented. This was a major point of discussion for PR #16605

Describe the solution you'd like

I would like documentation to be written for the GNSS lowerhalf driver so that users writing GNSS uORB drivers in the future are made aware of its existence and also how to integrate it into their driver.

This way we can actually expect users to use it.

Describe alternatives you've considered

No response

Verification

• I have verified before submitting the report.

[linguini1]

@Donny9, I believe you wrote this lower-half implementation. Would you be able to write some short RST documentation about it?