fix: escape pipe characters in Markdown values table cells

[SAY-5]

Proposed changes

A pipe character in a values.yaml value (or in a # -- description) is currently emitted verbatim into the Markdown values table, where it is parsed as a column separator and breaks the row layout. This escapes | as \| when rendering the Default and Description cells of the Markdown table. Closes #171

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

The escape is applied only in the *RenderMd column templates, so the HTML output renderer is unaffected.