Tables: add table directive with column width control

[reakaleek]

What

• Add {table} directive that wraps pipe tables and controls column widths via a 12-unit grid (similar to Bootstrap)
• Support presets: auto (default, browser determines widths from content), description (4-8 for term/description)
• Support custom widths: dash-separated values that sum to 12 (e.g. 4-8, 2-4-6, 4-4-4)
• Inject <colgroup> and table-layout:fixed when widths are specified so browsers respect column proportions

Why

• Authors need to control table column widths for definition lists, API parameter tables, and asymmetric layouts
• Without explicit widths, the browser sizes columns from content, which often looks wrong for narrow keys vs wide descriptions

Notes

• Column count validation runs at render time, not during block close, because Markdig converts pipe tables from paragraphs to table blocks during inline parsing (after the directive block is closed)
• Uses invariant culture for decimal formatting in colgroup so widths render consistently across locales

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/tables.md