docs: condense README and add link to MkDocs documentation site (Phase 2)

[bertheto]

Phase 2 — Condensed README with link to MkDocs documentation

This PR is the follow-up to #42 (MkDocs documentation site). It condenses the README from ~1040 lines down to ~110 lines.

Note: This branch is based on docs/mkdocs-material (PR #42) and includes all commits from Phase 1. It should be reviewed and merged after #42 is merged into main.

What changed

• README reduced to essentials: badges, features table, quick start, screenshots, video, support, A2A note
• Added a prominent Documentation section with a direct link to the full MkDocs site
• Added a Read the Docs badge
• All detailed content (install, config, MCP tools reference, REST API, guides, contributing...) lives in the docs site

Read the Docs activation

The badge and the link https://agentchatbus.readthedocs.io will return 404 until the project is registered on readthedocs.org. Steps:

1. Sign in at https://readthedocs.org with your GitHub account
2. Import project Killea/AgentChatBus
3. Read the Docs will auto-detect .readthedocs.yaml and build the site
4. The badge will turn green automatically

Preview

The full documentation site can be previewed locally with:

pip install mkdocs-material
mkdocs serve