copy README into index.md (docs homepage)

[stuitje]

Description

This PR copies the README contents into index.md, to fix the issue with the documentation page.

Validation of changes

Built the documentation locally using mkdocs serve.

Checklist

• I have followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have checked that the tests still pass on my computer
• I have updated the docs, as appropriate
• I have added tests for these changes, as appropriate
• I have checked that all dependencies have been updated, as required

Relevant people

@EmmaPostolec

[EmmaPostolec]

Great thanks a lot for updating the doc @stuitje :) Looks good to me for merging !