[Docs] Fix landing page of collections documentation

[Azoy]

Docc doesn't really like reexported modules, so the Collections default target couldn't access any of the other symbols from its documentation. Provide relative links to those instead and add some blurbs about the new RigidArray and UniqueArray.

[lorentey]

 Linking swift-collectionsPackageTests
Build complete! (27.26s)
error: Exited with unexpected signal code 5
The bundle “swift-collectionsPackageTests.xctest” couldn’t be loaded. Try reinstalling the bundle.
(dlopen(/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/swift-collectionsPackageTests, 0x0109): Library not loaded: @rpath/libswiftCompatibilitySpan.dylib
  Referenced from: <9DBEEAD4-5931-3932-BB6E-CD0B65BAEEA5> /Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/swift-collectionsPackageTests
  Reason: tried: '/Applications/Xcode_26.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/libswiftCompatibilitySpan.dylib' (no such file), '/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/../../../libswiftCompatibilitySpan.dylib' (no such file), '/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xcte<…>)
swiftpm_testing_helper/Entrypoint.swift:34: Fatal error: Failed to open test bundle at path /Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/swift-collectionsPackageTests: dlopen(/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/swift-collectionsPackageTests, 0x0101): Library not loaded: @rpath/libswiftCompatibilitySpan.dylib
  Referenced from: <9DBEEAD4-5931-3932-BB6E-CD0B65BAEEA5> /Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/swift-collectionsPackageTests
  Reason: tried: '/Applications/Xcode_26.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/libswiftCompatibilitySpan.dylib' (no such file), '/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/../../../libswiftCompatibilitySpan.dylib' (no such file), '/Users/runner/actions-runner/_work/swift-collections/swift-collections/.build/arm64-apple-macosx/debug/swift-collectionsPackageTests.xctest/Contents/MacOS/../../../libswiftCompatibilitySpan.dylib' (no such file), '/usr/lib/swift/libswiftCompatibilitySpan.dylib' (no such file, not in dyld cache), '/System/Volumes/Preboot/Cryptexes/OS/usr/lib/swift/libswiftCompatibilitySpan.dylib' (no such file), '/usr/lib/swift/libswiftCompatibilitySpan.dylib' (no such file, not in dyld cache), '/System/Volumes/Preboot/Cryptexes/OS/usr/lib/swift/libswiftCompatibilitySpan.dylib' (no such file)