feat: strip "$ " shell prompts from copy button

[iamsharduld]

Issue

Copying a $ command block from our docs pastes the $ along with it, which then fails when run in a real shell. Affects every shell code block across the site.

Solution

Adds two lines to docs/conf.py configuring sphinx-copybutton to strip the $ prompt on copy. Also sets copybutton_line_continuation_character = "\\" so multi-line commands joined by \ copy as a single command.

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.

Context

sphinx-copybutton (auto-enabled by canonical_sphinx) supports prompt-stripping but doesn't apply it by default. With copybutton_prompt_text = r"\$ " and copybutton_prompt_is_regexp = True, blocks containing $ -prefixed lines copy only the command portion, and blocks without prompts (e.g. JSON output) copy verbatim.

Aligns with the [Canonical Documentation Style Guide](https://docs.ubuntu.com/styleguide/en/), which advises against prompt marks in code samples for this exact reason.

Testing Instructions

1. cd docs && make clean && make serve
2. Open any tutorial page, find a $ shell block, click the copy button.
3. Paste somewhere — should yield command not $ command.
4. Verify a JSON or output-only block still copies verbatim.

Upgrade Notes

None — additive config-only change. No content or build-pipeline changes.

[mmkay]

Good job!

[MichaelThamm]

FYI this is a comment from Yanisa, who was our temp technical author

• No prompts

Removing it from copy is helpful if prompts are there. IMO I don't like them mostly because the main argument for having them is to denote what is the input vs what is the output, but this encourages poor practices of just copying large sections of the terminal w/o sufficient explanation anyway