add add_agent.md

[jaybutera]

Adds a self-documenting microagent that helps users create new microagents with proper structure and best practices. This makes it easier for users to extend OpenHands' capabilities by adding their own domain-specific agents.

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

Release Note: Added a new microagent template system that helps users create properly structured domain-specific agents. Simply mention "new agent" or "new microagent" to easily add a repository agent and extend OpenHands' capabilities.

Summary

Creates add_agent.md microagent that explains:

• What is a microagent
• Where to put it in the local repository
• Links to microagent documentation and examples

Example Usage

create an agent for google docs with my api key: <key>
create an agent to interface with codeburg