Add an icon in the browser tab

[wreise]

Reference issues/PRs None.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description Add a favicon (icon in the browser tab).

Screenshots (if appropriate) New look:

Checklist

• [X] I have read the guidelines for contributing.
• [X] My code follows the code style of this project. I used flake8 to check my Python changes.
• [X] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [X] All new and existing tests passed. I used pytest to check this on Python tests.