Plan parsing and conversion notebook

[Framba-Luca]

This PR:

• Adds the accepted plan format to the parse_plan documentation
• Adds a notebook that shows how to parse a plan from a string and how to convert a plan from a format to another

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6caa639) 85.03% compared to head (1ad2073) 85.03%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #488   +/-   ##
=======================================
  Coverage   85.03%   85.03%           
=======================================
  Files         198      198           
  Lines       26053    26053           
=======================================
  Hits        22153    22153           
  Misses       3900     3900           

Files Changed	Coverage Δ
unified_planning/io/pddl_reader.py	79.00% <ø> (ø)

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