Conform LocalStack README to merged docs-parity checks

Main's new tests/test_docs_parity.py requires each capability README to link
its own source module and use a spaced/allow-listed H1. Add the Source link and
allow-list the LocalStack proper noun (same escape hatch as FileSystem).

pydantic_ai_harness/experimental/localstack/README.md

@@ -16,6 +16,8 @@
 Give an agent access to an emulated AWS environment, so it can provision and
 exercise AWS services without touching a real account.
 
+[Source](https://github.com/pydantic/pydantic-ai-harness/tree/main/pydantic_ai_harness/experimental/localstack/)
+
 ## The problem
 
 Agents that build or test cloud infrastructure need somewhere to create buckets,

tests/test_docs_parity.py

@@ -96,7 +96,7 @@ _EXPERIMENTAL_MARKERS = ('HarnessExperimentalWarning', 'removed in any release',
 # Lifecycle hook names must not lead a page -- mechanism goes below the purpose.
 _LEAD_HOOK_NAMES = ('before_model_request', 'after_model_request', 'before_tool_execute', 'after_tool_execute')
 # ClassName-style headings are a smell, except where the class name IS the name.
-_ALLOWED_CLASSNAME_HEADINGS = {'FileSystem'}
+_ALLOWED_CLASSNAME_HEADINGS = {'FileSystem', 'LocalStack'}
 _FORBIDDEN_HEADINGS = {'overflow', 'authoring', 'overflow capability', 'compaction capabilities'}