Add descriptions and a diagram to docs documenting vertical offsets (water level, etc)

[emiliom]

From @leewujung : For the sake of clarity, could you add the definition of water_level and vertical_offset with a simple diagram of which one means which for an underwater vehicle and a ship? This should actually be in the docs, but if we have something parked here it'll be easy to plug them in afterwards.

[emiliom]

Refer to #1051 for related discussions.

[leewujung]

In #1093 I've restructured the TOC to have a Data processing section with 2 sections:

• data-proc-func.ipynb: providing a summary of processing functionalities and usage examples, especially for cal_params and env_params for calibration
• data-proc-additional.ipynb: a one-stop-shopping place for additional details for processing functions

One could argue that the cal_params and env_params details should be in the -additional page, but since I feel the -func page could also use some more expansion, I decided to hold off on any further changes.

I think the -additional page may be the place to go for the z-axis variables and combinations, and to include an illustration.

[leewujung]

This is now addressed in #1133!