Add documentation for Trusty PR check rule (#1508)

docs/docs/ref/policies/activity.md

@@ -3,4 +3,33 @@ title: Activity Score
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-# Activity Score Threshold Policy
+# Activity Score Threshold Policy
+
+The following rule type is available for [Trusty](https://www.trustypkg.dev/) activity score threshold.
+
+## `pr_trusty_check` - Verifies that pull requests do not add any dependencies with low Trusty scores
+
+This rule allows you to monitor new Pull requests for newly added dependencies with low
+[Trusty](https://www.trustypkg.dev/) scores.
+For every pull request submitted to a repository, this rule will check if the pull request adds a new dependency with
+a low Trusty score. If a dependency with a low score is added, the PR will be commented on.
+
+## Entity
+- `pull_request`
+
+## Type
+- `pr_trusty_check`
+
+## Rule Parameters
+- None
+
+## Rule Definition Options
+
+The `pr_trusty_check` rule has the following options:
+
+- `action` (string): The action to take if a package with a low score is found. Valid values are:
+  - `summary`: The evaluator engine will add a single summary comment with a table listing the packages with low scores found
+  - `profile_only`: The evaluator engine will merely pass on an error, marking the profile as failed if a packages with low scores is found
+- `ecosystem_config`: An array of ecosystem configurations to check. Each ecosystem configuration has the following options:
+  - `name` (string): The name of the ecosystem to check. Currently `npm` and `pypi` are supported.
+  - `pi_threshold` (number): The minimum Trusty score for a dependency to be considered safe.