Add agent to foundational agents documentation

Problem to solve

We should create documentation describing:

1. What the agent is for
2. What the agent does
3. How to set it up
4. Example questions to ask it
5. How to provide feedback

Things we need to be able to verbalize and include in docs are:

• What user and/or customer problems are we trying to solve?
• Example workflows
• Example prompts (https://docs.gitlab.com/user/duo_agent_platform/agents/foundational_agents/security_analyst_agent/#example-prompts) that we've optimized the agent for in this iteration
• Customer value

See examples of other foundational agents under: https://docs.gitlab.com/user/duo_agent_platform/agents/foundational_agents/