docs: add comprehensive v0.3 to v1.0 migration guide (#987)

# Description

This PR adds detailed documentation for migrating A2A-compliant
applications from version v0.3 to v1.0. The guide covers the transition
to full A2A Protocol v1.0 compatibility, including major architectural
shifts and developer experience improvements.

###  Key Areas Covered:
* **Dependency Management**: Instructions for upgrading to
a2a-sdk>=1.0.0 using uv or pip.
* **Type System Transition**: Detailed mapping of the move from Pydantic
models to Protobuf-based classes, including the standardization of enum
values to `SCREAMING_SNAKE_CASE`.
   * **Server-Side Refactoring**:
* Transition from application wrappers (A2AStarletteApplication, etc.)
to flexible route factory functions.
       * Updated DefaultRequestHandler signature requiring agent_card.
   * **Client Improvements**:
* New create_client() factory function replacing the legacy A2AClient
class.
* Standardization of send_message() return types to
AsyncIterator[StreamResponse].
* **Backward Compatibility**: Strategies for running v1.0 servers that
simultaneously support v0.3 clients during transition periods.
* **New Helper Utilities**: Introduction of the a2a.helpers module to
simplify object construction and data extraction.

### Why this is important:

The v1.0 release introduces several breaking changes to align with the
latest protocol specification and improve performance through Protobuf
integration. This guide is essential for existing users to successfully
navigate the upgrade path while minimizing downtime.

---------

Co-authored-by: Sampath Kumar <sam1990kumar@gmail.com>

Author: sokoliva

.github/actions/spelling/allow.txt

@@ -85,6 +85,7 @@ lifecycles
 linting
 Llm
 lstrips
+mcp
 middleware
 mikeas
 mockurl