Add intro to agents doc

[luisquintanilla]

This pull request adds a new conceptual documentation page introducing agents and workflows in the context of .NET AI development. It also updates the table of contents to include this new page, making it easier for users to discover information about building agents.

Documentation additions:

• Added a new conceptual documentation page docs/ai/conceptual/agents.md that explains what agents are, their capabilities, the role of workflows, and how to get started building agents in .NET using Microsoft.Extensions.AI and related SDKs.

Documentation navigation improvements:

• Updated the AI documentation table of contents (docs/ai/toc.yml) to include a link to the new "Building Agents in .NET" page.

Fixes #48047.

---

Internal previews

📄 File	🔗 Preview link
docs/ai/conceptual/agents.md	Agents
docs/ai/index.yml	[linkListType: architecture
docs/ai/toc.yml	docs/ai/toc

[luisquintanilla]

Trigger CI

[gewarren]

For the build failure, it looks like there was previously a file with the same name that was removed, so you probably just need to remove that redirect.

[luisquintanilla]

@gewarren looks like all builds are passing. Is there a way to render mermaid diagrams in docs?

This is what I see in preview -

[gewarren]

Is there a way to render mermaid diagrams in docs?

No, I don't think so :( :( cc @IEvangelist who has also tried this.

[luisquintanilla]

@gewarren no worries! Whenever you get a chance, this is ready for review and approval. Thanks!