docs: Update plugin loading documentation for Conversation-based approach

[jpshackelford]

Summary

Update documentation to reflect the actual plugin loading implementation in software-agent-sdk#1651.

Changes

• Update "Recommended" section to use plugins parameter on Conversation (not AgentContext.plugin_source)
• Use PluginSource model for specifying plugins
• Update example path from 03_plugin_via_agent_context to 03_plugin_via_conversation
• Update Quick Reference section
• Fix anchor link in Manual Plugin Loading note

The Recommended Pattern

from openhands.sdk.plugin import PluginSource

conversation = Conversation(
    agent=agent,
    workspace="./workspace",
    plugins=[
        PluginSource(source="github:org/security-plugin", ref="v2.0.0"),
        PluginSource(source="github:org/monorepo", repo_path="plugins/audit"),
        PluginSource(source="/local/custom-plugin"),
    ],
)

Related

• feat(agent-server): Support plugin loading when starting conversations software-agent-sdk#1651 - Plugin loading support in agent-server

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • .github/workflows/sync-agent-sdk-openapi.yml
  • .github/workflows/sync-docs-code-blocks.yml

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #265 at branch `feat/plugin-via-agent-context-example`

Feel free to include any additional details that might help me get this PR into a better state.

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