docs: Add MCP SDK documentation and architecture diagrams

Add comprehensive documentation for the MCP Java SDK implementation
Add architecture diagrams showing layered design and class relationships
Reorganize documentation structure with dedicated pages for:
Overview and introduction
MCP Java SDK reference
Spring AI MCP integration
Include detailed examples and usage patterns for both sync and async APIs

Author: tzolov

mcp-docs/src/main/antora/modules/ROOT/images/MCP-layers.svg

@@ -1,3 +1,5 @@
-* xref:index.adoc[Overview]
+* xref:overview.adoc[Overview]
+* xref:mcp.adoc[MCP Java SDK]
+* xref:spring-mcp.adoc[Spring AI MCP]
 // ** xref:concepts.adoc[AI Concepts]
 // * xref:getting-started.adoc[Getting Started]