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.