Write docs based on the four pillars of documentatoin

[felixcremer]

This is a nice explanation of the four functions of documentation:
https://documentation.divio.com/introduction/
The four parts are:
Tutorials
How-to guides
Reference
Explanation

From our discussion earlier, the next steps for the documentation are:

• Write a tutorial @meggart
• Write a how to use Tiff files (or arbitrary files from GDAL) in YAXArrays @felixcremer
• Exposing the docstrings as a reference guide @felixcremer See Add an api reference to docs #41
• Write a performance tips section to explain the influence of chunking and parallel computation on the performance. @meggart

I'd like to use this issue to track the development of the documentation.

[felixcremer]

Check for public tiff files to write the gdal tutorial.
Maybe use public data used by https://github.com/e-sensing/sits

[lazarusA]

new docs, cover most of these things if not all.