Documentation about the structure tree and name convention of GFs database

[Jinyin-Hu]

Hello,

There might be a need to include some descriptions about the preparation of Green's functions (GFs) database. In the online documentation, I only find some sentences about how to read the GFs database from FK or CPS etc. But I didn't find the structure layer/tree and name convention of the database for FK or CPS method. I think it may be very helpful for new users like me to learn this information without going into the code. For example, in the code, mtuq.io.clients.CPS_SAC.py, I found this information:
'''
GreensTensors are obtained by reading precomputed time series from an
CPS directory tree with naming convention ZZZz/RRRRrZZZz.EXT,
where ZZZz gives the depth of the source, RRRRr gives the horizontal offset
between source and receiver, and EXT is the file extension related to
so-called fundamental sources.

.. note::

  The above directory tree convention permits us to represent offsets 
  from 0 to 9999.9 km in 0.1 km increments and origin depths from 
  0 to 999.9 km in 0.1 km increments

'''

They are important to know when preparing the CPS database, and it would be better to include them in the documentation.

Many thanks.