Add quickstart to readme

[juanitorduz]

• Adding quickstart and some plots to make the README more engaging.
• Added some additional features for the mmm module.
• Added PyMC Logo closer to the start (and link to contact and services)
• Community links

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📚 Documentation preview 📚: https://pymc-marketing--653.org.readthedocs.build/en/653/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.73%. Comparing base (5a93ef2) to head (31a33c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #653   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files          22       22           
  Lines        2322     2322           
=======================================
  Hits         2130     2130           
  Misses        192      192           

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