Add Docs on Supported Metrics

[shangyian]

Summary

Expands the metrics documentation with more complete coverage of all supported aggregation functions and derived metric patterns.

New Sections Added

• Supported Aggregation Functions - Documents all aggregation functions with examples:
  • Fully aggregatable: SUM, COUNT, COUNT_IF, MIN, MAX, AVG, ANY_VALUE, APPROX_COUNT_DISTINCT
  • Statistical: VAR_POP, VAR_SAMP, STDDEV_POP, STDDEV_SAMP, COVAR_POP, COVAR_SAMP, CORR
  • Non-aggregatable: MAX_BY, MIN_BY
  • DISTINCT aggregations and their limited aggregability
• Derived Metrics - Covers supported patterns:
  • Simple derived metrics (arithmetic combinations)
  • Cross-fact ratio metrics (numerator/denominator from different facts)
  • Period-over-period metrics (WoW, YoY using conditional aggregation or window functions)
  • Trailing N-day metrics (rolling windows)
  • Share of total metrics (with clear note about limited support and link to Support Multi-Scope Ratio Metrics #1695)
• Conditional Aggregations - Using CASE WHEN inside aggregations
• Metric Metadata - Documents required_dimensions, direction, and units

Other Changes

• Fixed "Define Metrics Using SQL" link on landing page to point to metrics docs
• Fixed YAML docs link in overview.md

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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