DOC: create preprocessing section

[martinfleis]

This creates a preprocessing section in the user guide and moves and renames existing preprocessing notebook. I have also added redirection from the old location of the notebook to the new one, though I am not sure it works now :). Want to see the RTD build.

@gregmaya @gsagostini please put your notebooks in this section once this gets merged.

[martinfleis]

I have also added redirection from the old location of the notebook to the new one, though I am not sure it works now :)

It does not, even though it is 1:1 copy from geopandas. Will need to debug.

[jGaboardi]

Two things for the simple_preprocessing notebook:

• Can we rerun for consecutive cell numbering?
• Correct spelling in text above cell 29:
  • "... in this situation as we are nor -> not snapping endpoints to endpoints."

[jGaboardi]

@martinfleis is this ready to merge or does it still need debugging as you mentioned above?

[martinfleis]

I still need to figure out the redirect. Working on it right now.

[martinfleis]

Fixed, merging.