(fwData): add a documentation about the data

Description

The sight data are not documented. Write a section about Image and mesh usage (allocation, parsing, ...).

Proposal

Add a global data documentation in the SAD and improve the tutorials (Filter and/or Mesher) to explain how to parse the Image and Mesh with the new API.

Links / references

See sight!285 (merged), sight!273 (merged) and sight!254 (merged)