Improve documentation

[remi-braun]

• Update Contributing page with building documentation section
• Add documentation about writing into BytesIO Allow rio.to_raster with a BytesIO object #668, solves also DOC: add section on writing files in docs #584 about writing raster in the cloud
• Add a page dedicated to the switch from rasterio to rioxarray, and notably equivalences between rasterio functions and rioxarray methods
• Add a page about GDAL configuration options Does rioxarray use internal GDAL config parameters? #693
• Add and initialize external courses toctree
  • Carpentries courses DOC: link geospatial-python course in the docs #279
  • Geospatial Python courses by Earth Lab at University of Colorado, Boulder

[remi-braun]

I tried to fix some open documentation issues and propagate some user questions.
This can of course be amended by anybody, it's more of a draft right now.

Other questions :

• do you want to switch to another sphinx them?
• do you want to switch to markdown files instead of rst?

[remi-braun]

For the rasterio / rioxarray equivalences, it could be great to explain why some attributes in rasterio are becoming methods when others don't (i.e. transform). It's to preserve the lazy evaluation inherited from xarray and not recomputing on the fly these attributes when changing the raster (i.e. cropping, reprojecting...)?