docs: add post calculation metrics

[ZeRego]

Related to: lightdash/lightdash#17437
Related to: https://linear.app/lightdash/issue/CENG-77/support-percent-of-total-and-other-post-sql-metric-types-in-looker2dbt

Add Post Calculation Metrics

This PR introduces post calculation metrics, a new experimental feature that allows for computations after aggregate and non-aggregate metrics in a query. Post calculation metrics enable window functions and include three new metric types:

1. percent_of_previous - Shows the current value as a percentage of the previous row's value
2. percent_of_total - Calculates the current value as a percentage of the total across the result set
3. running_total - Computes a cumulative total

Post calculation metrics:

• Can only reference aggregate and non-aggregate metrics
• Can be referenced in table calculations
• Do not support the filters YML property

Examples for each new metric type are included in the documentation with sample YAML configurations.

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[ZeRego]

• docs: add post calculation metrics #183 👈 (View in Graphite)
• main

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[linear]

CENG-77 Support percent_of_total and other Post-SQL metric types in looker2dbt and Lightdash