docs(fix[copybutton]): Enable line continuation for multiline console blocks

why: sphinx-copybutton's copy button silently drops continuation lines in
multiline `\`-continued console commands. For example, clicking copy on:

```console
$ pipx install \
    --suffix=@next \
    --pip-args '\--pre' \
    --force \
    'libtmux'
```

Only copies `pipx install \` — the continuation lines are discarded.

Root cause: `copybutton_line_continuation_character` defaults to `""`.
In copybutton_funcs.js, `useLineCont = !!""` evaluates to `false`, so
`gotLineCont` never activates. Lines not matching the prompt regex (`$ `)
are silently dropped by the formatCopyText() line-processing loop.

Setting it to `"\\"` makes `useLineCont = true`, and any line ending in
`\` sets `gotLineCont = true` for the next iteration, including it
regardless of prompt match.

This is the standard fix adopted by Apache Arrow, NVIDIA TensorRT-LLM,
Flower, Crate docs theme, and Nextstrain sphinx theme.

Refs:
- https://sphinx-copybutton.readthedocs.io/en/latest/use.html
- executablebooks/sphinx-copybutton#126
- executablebooks/sphinx-copybutton#65
- executablebooks/sphinx-copybutton#52

what:
- Add `copybutton_line_continuation_character = "\\"` to docs/conf.py

Author: tony

docs/conf.py

@@ -143,6 +143,7 @@
 )
 copybutton_prompt_is_regexp = True
 copybutton_remove_prompts = True
+copybutton_line_continuation_character = "\\"
 
 # sphinxext-rediraffe
 rediraffe_redirects = "redirects.txt"