Issue #181: Don't delete README on make clean

[Mark2000]

Description

Closes #181

Please include a summary, motivation, and context of the changes and the related issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How should this pull request be reviewed?

• By commit
• All changes at once

How Has This Been Tested?

N/A, tested building docs

Checklist:

• My code follows the style guidelines of this project (passes Black, ruff, and isort)
• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation and release notes
• Commit messages are atomic, are in the form Issue #XXX: Message and have a useful message
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• If I changed an example ipynb, I have locally rebuilt the documentation

[LorenzzoQM]

Looks good.