[Docs]: OASF: update core feature list

[jodeev]

Issue Description

OASF supports A2A cards, MCP servers, agent dependencies on MCP server tools, etc.

This is currently buried deep in the OASF schema and needs calling out in the core docs.

Prposed Solution

Update this page https://docs.agntcy.org/oasf/open-agentic-schema-framework/ to outline supported functionality, and how records are constructed for different use cases (OASF core, A2A cards, MCP servers, LLMs, other dependencies, etc), referring to other pages (like https://docs.agntcy.org/oasf/model.proto/#agent-diagram, and correct reference examples) as needed.

It should cover the core record and how extensions are used, to fill in the gaps of the other existing documentation.

Affected Component

CSIT

Checklist

• I have read the contributing guidelines.
• I have verified this does not duplicate an existing issue.