Add sphinx_copybutton extension to allow copying from code blocks

[nathandyer]

I'm filing this PR so we can begin a discussion about whether or not this is something we want to include in our docs.

As part of the docs re-org and the new Book theme (🙏️ @ChumOfChance), it was previously mentioned that it would be really nice if we could include a copy button within code blocks in our docs. This PR enables that functionality via the sphinx-copybutton extension.

For example:

Although this extension hasn't seen an update in a few years, I think that's because it's a relatively minimal extension with only a small amount of Javascript + CSS to make it work. I'm not sure there's really that much to update or maintain.

This is nice because for people who are setting up SecureDrop from a daily driver system with Javascript enabled, this provides copy/paste functionality for longer commands. And in environments where Javascript is not enabled, this fails gracefully (there's just not a copy/paste button displayed).

Is this something we might want to consider?

Test plan

• CI is happy
• Visual review
• Discussion outcome is that the team is OK with moving forward with these changes

Checklist

This change accounts for:

• local preview of changes beyond typo-level edits

[legoktm]

+1 to adding the functionality, neutral on the package itself vs writing/forking/vibecoding our own, clipboard.js seems pretty overkill for what we need here (but no actual issues other than it using deprecated functionality).

[ChumOfChance]

I'm +1 for this. Big quality of life improvement. I would err on whatever solution is the easiest to integrate, so an old reliable extension seems best! I have not looked at it in a local build, but I'd want to make sure it looks OK on different sized devices, doesn't bug out on any unusual code blocks, etc.

[zenmonkeykstop]

extension itself looks good! If possible, enabling its prompt detection features to not display it for non-command-line snippets (eg the gpg output in /admin/installation/prepare_sdw.html), and display it only for cut-n-pasteable commands would be cool. (It also needs a rebase.)

[nathandyer]

Thanks @zenmonkeykstop! I've taken another pass here, and have disabled the copy button on blocks that should not be copy-and-pasted from. This should be ready for another review.

I'll wait to hear your feedback from this approach before doing the same in the related dev docs PR.