Migrate from sphinx-panels to sphinx-design

[yousefmoazzam]

Fixes #470
Fixes #469

Checklist

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation

[yousefmoazzam]

During local testing, the dropdown colour in dark mode had no readability issues, regardless of if the general.css file had its contents, or was empty.

Therefore, the dropdown seems to be readable in dark mode regardless of if the file is there or not, hence, it doesn't seem to be needed, which is why I have removed it.

[yousefmoazzam]

Also note that the latest docs build job indicates sphinx v8.1.3 was installed:

Running Sphinx v8.1.3

So, I think this PR fixes #469 as well.