docs: Restrict copy-paste to input code

[savsch]

Resolves #106

This PR attempts to follow the method mentioned in the linked issue to support restricting copy-pasting of content inside code blocks (i.e. inside ```) to just input code in the following manner:

• Lines starting with the prefix $ are considered to be "input lines". They can be copied.
• Lines starting with the prefix << ("output lines") or # ("comment lines") are neither copied using the copy button, nor can they be selected by the user.

Also, the prefixes ($ , << , # ) themselves cannot be copied or selected.

Screenshots

This is what one of the changes looks like:

Before	After

Behaviour of the copy-to-clipboard button:

docs_restrict_copying_to_input_code.mp4

[iaine]

Hi savsch,

Thanks for your PR.

If this is for the GSoC tests, could you please add a screenshot for the Hard1 test and a link to this PR on the wiki page:
https://github.com/rstats-gsoc/gsoc2025/wiki/Refining-the-R-Dev-Container

[hturner]

Thanks for the PR. Unfortunately, we had someone investigate this approach earlier and although the conclusion was not to pursue it we had not updated #106 (there was a link to the related discussion, r-devel/r-dev-day#60, but we had not posted a conclusion). I've now done this and closed both issues, so will also close this PR. For the purposes of GSoC though, this is still a valid submission for the Hard Test 1.