add logo

[jlaehne]

Description of the change

Adds the logo as discussed in #20 and already used on https://hyperspy.org

Closes #20

Progress of the PR

• Change implemented (can be split into several points),
• update user guide (if appropriate),
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:holospy build of this PR (link in github checks)
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (7ec7067) to head (3985e03).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files           7        7           
  Lines         330      330           
  Branches       73       73           
=======================================
  Hits          288      288           
  Misses         25       25           
  Partials       17       17           

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