feat: mpe_validations table and a note about the pgmloc var in hook scripts

docs/tables/mpe_validations.md

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+---
+layout: article
+title: MPE_VALIDATIONS
+description: The `MPE_VALIDATIONS` table enables a number of validations to be applied to the data at the point of entry - such as casing, min/max values, nullability, and the ability to generate dropdown values directly from source tables or dynamically from SAS programs.
+og_image: /img/mpe_validations.png
+---
+
+# MPE_VALIDATIONS
+
+The `MPE_VALIDATIONS` table enables a number of validations to be applied to the data at the point of entry - such as casing, min/max values, nullability, and the ability to generate dropdown values directly from source tables or dynamically from SAS programs.
+
+A detailed breakdown is available in the [validations](/dc-validations/) section.
+
+![validations](/img/mpe_validations.png)
+
+## Columns
+
+ - 🔑`TX_FROM num`: SCD2 open datetime
+ - `TX_TO num`: SCD2 close datetime
+ - 🔑 `BASE_LIB char(8)`: SAS Libref (8 chars)
+ - 🔑 `BASE_DS char(32)`: The library member name
+ - 🔑 `BASE_COL char(32)`: The column name
+ - 🔑 `RULE_TYPE char(32)`: The name of the rule to apply.
+ - `RULE_VALUE char(128)`: The value of the rule
+ - `RULE_ACTIVE num`: Set to 1 for an active rule.  Set to 0 to disable the rule.