nrf_modem: doc: Change GNSS interface name

[tokangas]

Changed GNSS interface name in documentation from Global Satellite Navigation System to GNSS. The name of the interface in modemlib is GNSS and in general all documentation refers to GNSS interface or GNSS API.

[tokangas]

@eivindj-nordic @lemrey The title of the gnss_interface.rst was changed in a dectphy PR (#1325), which I have missed because I wasn't asked for a review. I noticed this now, because I added a reference to it.

I created this PR to change the name from "Global Navigation Satellite System" to "GNSS", but actually I think the name should be reverted back to what it was earlier, i.e. "GNSS interface". The page describes the GNSS interface/API, not GNSS. All references to that page are also affected. For example, the meaning here has changed:

https://github.com/nrfconnect/sdk-nrf/blob/741cb7aa25da073966de033ca9d79788e2fc6a0b/doc/nrf/libraries/modem/location.rst?plain=1#L35

That used to be "Uses GNSS interface for getting the location", but now it says "Uses Global Navigation Satellite System for getting the location". That is not the same thing.

I guess the purpose of the change was to align the naming, but I don't think the current way is good. I don't know if there are similar problems with references to the other interface documents.

[eivindj-nordic]

Looking at where it is used in documentation I think it should either be changed to GNSS interface, or we should update the documentation where it is used to add interface after the reference. I don't have a strong opinion of what is best.

[tokangas]

We had a call with Emanuele and agreed that we'll go with "GNSS". I'll update the references in the SDK.