Add shell block execution with captured output rendering

[mwittie]

Summary

Add support for executing shell commands from Markdown code blocks during the documentation build and rendering the captured output back into the document.

Problem / Motivation

Documentation examples often show commands that can drift from actual behavior over time. If shell blocks can be executed at build time, the documentation can show verified command output rather than manually copied output.

This helps make documentation a checked, executable artifact rather than static prose.

Proposed Solution

Support an executable shell block syntax and output like:

```bash | litdoc -l text
echo "hello, world"
```

<!--- BEGIN LITDOC OUTPUT -->
```text
hello, world
```
<!--- END LITDOC OUTPUT -->

Acceptance Criteria

• Shell code blocks can be marked for execution
• Captured stdout can be rendered into the generated document
• Captured stderr handling is defined and documented
• The original command remains visible to readers unless separately suppressed
• Re-running the build is idempotent and does not duplicate captured output