ENH: Add data reader for NIRx NIRSport2 NIRS files

[rob-luke]

I now have access to a NIRSport2 NIRx device. So I will take some small test measurements to ensure our read_raw_nirx function can support it. Following #9348 I don't think this will take much code change.

I will also tidy up the https://mne.tools/stable/auto_tutorials/io/30_reading_fnirs_data.html tutorial as it is growing (#9355) and could now do with a general intro and more details on how fNIRS is handled in MNE. Just an FYI so work isn't duplicated.