Create an appendix to API Design Guide

[rartych]

Problem description
The pragmatic approach for MCP related API guidelines in Sync26 is as follows:

• creating an appendix to API Design Guide defining the direction for MCP Enablement in API definitions that could become normative in Sync27
• starting with a few widely adopted APIs (pilot APIs) in Sync26 implementing additional guidelines proposed in the appendix
• preparing updated API Design Guide and Tool Definition Guide within Signal27 cycle - to be implemented in Sync27

Expected action
Prepare the appendix to API Design Guide based on previous discussions in #623 and #624

Additional context