docs: update AgentRegistry API reference

[thunderisme]

Summary

• Update the AgentRegistry documentation to match the current implementation.
• Correct the stale add(agent_id, agent) and ID-keyed examples; add now accepts an Agent and stores it by agent.agent_name.
• Replace the provider-heavy example with a concise local example covering add, list, get, query, update, and delete.
• Document the current constructor, method surface, and schema metadata fields.
• Add the minimal Read the Docs build config/support files used by recent passing docs PRs so the preview can build from this branch.

Bounty eligibility

• Label requested: bounty-eligible / 🙋 Bounty claim
• Expected tier: Silver or Gold, at maintainer discretion
• Rationale: this updates outdated AgentRegistry references and examples with behavior verified against swarms/structs/agent_registry.py; the RTD config files are included only to keep the docs preview buildable from this branch.

Verification

• Compared docs/swarms/structs/agent_registry.md against swarms/structs/agent_registry.py.
• Confirmed no open PR currently modifies docs/swarms/structs/agent_registry.md.
• Ran git diff --check.
• Confirmed recent docs PRs with the same RTD support pattern have passing Read the Docs checks.

[github-actions]

Hello there, thank you for opening an PR ! 🙏🏻 The team was notified and they will get back to you asap.

[thunderisme]

I could not apply the repository bounty label from the fork account. If this documentation update qualifies under the docs bounty program, please apply the appropriate bounty label during review. The bounty rationale is included in the PR description.