Improve repo presence: tools table, quick example, links

[adi-singh13]

What this PR does

Updates the README to make the repo more discoverable and useful for developers browsing GitHub/MCP registries.

Changes

• Tools section — Lists all 11 available MCP tools with descriptions. Registries like Glama pull from README to populate their listings.
• Quick example — 10 lines showing inbox creation + sending an email. Converts developers who are on the fence.
• Links section — One-click to docs, SDK, and main site.
• Updated intro — Clear one-liner: "Give your AI agent its own email address."

Still needs (repo settings, not PR):

1. Set repo description → "Give your AI agent its own email address. MCP server for AgentMail."
2. Set website URL → agentmail.to
3. Add topics → mcp, model-context-protocol, ai-agents, email, agentmail
4. All 3 founders Watch the repo → Zero watchers looks abandoned
5. Cut a v1.0.0 release after merging → 6 production deployments but zero releases looks experimental. Gets surfaced in "recently released" on MCP registries.

[cubic-dev-ai]

No issues found across 2 files