docs: Simplification of the DFtoVW tutorial

[etiennekintzler]

The previous tutorial on how to use the DFtoVW class (https://vowpalwabbit.org/docs/vowpal_wabbit/python/latest/tutorials/DFtoVW_tutorial.html) is too long and complicated (for what it tries to teach).

In this PR I created a version of the tutorial that is much shorter and does not depends on external dataset.

[bassmang]

@etiennekintzler looks good, except the Python docs CI seems to be failing, happy to merge when that is passing

[etiennekintzler]

@bassmang done :)