Enable the Docs soundness check (#1314)

Enable the (currently-disabled) Docs soundness check via GitHub Actions.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

.github/workflows/pull_request.yml

@@ -24,6 +24,6 @@ jobs:
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
     with:
       license_header_check_project_name: "Swift"
-      docs_check_enabled: false
+      docs_check_container_image: "swift:6.2-noble"
       format_check_enabled: false
       api_breakage_check_enabled: false

Sources/Testing/Testing.docc/Documentation.md

@@ -35,10 +35,8 @@ their problems.
 
 #### Related videos
 
-@Links(visualStyle: compactGrid) {
-  - <doc://com.apple.documentation/videos/play/wwdc2024/10179>
-  - <doc://com.apple.documentation/videos/play/wwdc2024/10195>
-}
+- [Meet Swift Testing](https://developer.apple.com/videos/play/wwdc2024/10179)
+- [Go further with Swift Testing](https://developer.apple.com/videos/play/wwdc2024/10195)
 
 ## Topics
 

Sources/Testing/Testing.docc/MigratingFromXCTest.md

@@ -556,9 +556,11 @@ test function with an instance of this trait type to control whether it runs:
   }
 }
 
-<!-- TODO: document Test.cancel() and Test.Case.cancel() here
+<!-- TODO: document Test.cancel() and Test.Case.cancel() here, and update
+     relevant links to use proper DocC symbol references.
+
 If a test has already started running and you determine it cannot complete and
-should end early without failing, use ``Test/cancel(_:sourceLocation:)`` instead
+should end early without failing, use `Test/cancel(_:sourceLocation:)` instead
 of [`XCTSkip`](https://developer.apple.com/documentation/xctest/xctskip) to
 cancel the task associated with the current test:
 
@@ -592,7 +594,7 @@ cancel the task associated with the current test:
 }
 
 If the test is parameterized and you only want to cancel the current test case
-rather than the entire test, use ``Test/Case/cancel(_:sourceLocation:)``.
+rather than the entire test, use `Test/Case/cancel(_:sourceLocation:)`.
 -->
 
 ### Annotate known issues