Document table calculation functions

[charliedowler]

Summary

• Add reference docs for 8 new table calculation functions from Lightdash PRs #19999 and #20102
• Row functions (row, offset, index, lookup, offset_list, list) — row-based functions that compile to SQL window functions
• Pivot functions (pivot_row, pivot_offset_list) — functions for accessing values across pivot columns
• Each function page includes signature, parameter table, practical example, and compiled SQL in an expandable accordion
• Add "Table calculation functions" nav group in the sidebar between the main table calculations page and SQL templates
• Cross-link from the table calculations guide and SQL templates overview

Test plan

• Verify both new pages render at /references/table-calculation-functions/row-functions and /references/table-calculation-functions/pivot-functions
• Verify "Table calculation functions" nav group appears in the sidebar
• Verify accordion blocks expand/collapse for compiled SQL sections
• Verify cross-links from table-calculations.mdx and sql-templates.mdx resolve correctly
• Verify existing SQL template pages are unaffected

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[torimwhaley]

LGTM 👍