Add "copy" button to the sphinx documentation page.

[chongchonghe]

Description

Following https://sphinx-copybutton.readthedocs.io/en/latest/ to add a little “copy” button to the right of the code blocks. Added sphinx_copybutton extension in conf.py.

Tested in local build (make html) and it works fine, but I'm not sure if the sphinx_copybutton extension is available on the server where the website HTML is built.

Checklist

Before this pull request can be reviewed, all of these tasks should be completed. Denote completed tasks with an x inside the square brackets [ ] in the Markdown source below:

• I have added a description (see above).
• I have added a link to any related issues see (see above).
• I have read the Contributing Guide.
• I have added tests for any new physics that this PR adds to the code.
• I have tested this PR on my local computer and all tests pass.
• I have manually triggered the GPU tests with the magic comment /azp run.
• I have requested a reviewer for this PR.

[BenWibking]

You can install additional any pip packages that you need to build the docs by editing this file:
https://github.com/quokka-astro/quokka/blob/development/.github/workflows/docs.yml

[chongchonghe]

You can install additional any pip packages that you need to build the docs by editing this file: https://github.com/quokka-astro/quokka/blob/development/.github/workflows/docs.yml

OK, that helps. Now every check should pass and it's ready for your review. @BenWibking

[BenWibking]

I don't think the docs render correctly. You can take a look at the output here (click to download the "github-pages" artifact): https://github.com/quokka-astro/quokka/actions/runs/8717957314?pr=609