docs: child context

- Rewrites child-context.md to be equally useful to TypeScript, Python,
  and Java developers
- Removes Table of Contents, back-links, Terminology, Key Features,
  Best Practices, and FAQ sections
- Adds walkthrough, method signature, ChildConfig, context function,
  naming, concurrency, and testing sections for all three languages
- Explains why child contexts are required for concurrent durable
  operations and the rules for using runInChildContext safely
- Adds new example files under examples/{typescript,python,java}/
  operations/child-contexts/

closes #71, closes #53, closes #35

Author: yaythomas

docs/getting-started/key-concepts.md

@@ -141,8 +141,9 @@ Wrap such non-deterministic code in [steps](../sdk-reference/operations/step.md)
 ### Rules for deterministic durable operations
 
 1. All durable operations in a context must start sequentially.
-2. To run durable operations concurrently, wrap each set of operations in its own child
-    context and then run the child contexts concurrently.
+2. To run durable operations concurrently, wrap each set of operations in its own
+    [child context](../sdk-reference/operations/child-context.md) and then run the
+    child contexts concurrently.
 3. Only use the child `DurableContext` in the child context scope. Do not use any
     parent's context in a child context scope.