docs: add Notion Write Toolkit integration

[dineshkumarkummara]

This PR follows LangChain’s external-integration docs pattern (e.g. Slack, Gmail). Maintainers only need to confirm formatting and link validity.

Summary

• Document the LangChain Notion Write Toolkit, a community-maintained package that lets agents both search and write to Notion using the official API, with both sync and async flows.
• Add comprehensive integration documentation under docs/integrations/tools/ covering installation, supported versions, authentication, sync/async usage, CLI usage, and full end-to-end agent examples (with accessible alt text).
• Highlight security/permission requirements (Notion scopes, payload sanitisation, rate limiting) plus current limitations and roadmap so reviewers understand the maintenance surface.

Why this matters to LangChain users

• Teams adopting LangChain frequently prototype copilots that summarise meetings, draft product updates, or triage support issues. Without a write-capable Notion integration, they must export results manually or maintain private one-off scripts. This PR closes that gap with a documented, production-ready path.
• The toolkit lives in an external repository (langchain-notion-tools), is MIT licensed, and is already on PyPI (pip install langchain-notion-tools==0.1.0). By officially documenting it, we give the community a discoverable, standardised way to persist agent output inside Notion workspaces.
• Keeping this contribution documentation-only means LangChain core stays lean while users gain the write functionality they have repeatedly requested in Discord, GitHub issues, and discussions. Reviewers can merge with low risk knowing maintenance happens in the upstream package.

Impact and community benefits

• Product teams can automate release notes, OKR updates, and decision logs directly in Notion without swapping tools.
• Support and operations teams can pipe summaries or ticket triage results straight into Notion databases, closing the loop between analysis and action.
• Developers and integrators gain a vetted example of how to package external LangChain tools with async support, CLI affordances, and strong input validation—helpful for future integrations.
• The docs explicitly call out constraints, token scopes, and roadmap items so community collaborators know where they can help next (e.g., attachments, synced blocks).

Testing & validation

• Markdown previewed locally; tables, code blocks, and the screenshot/GIF placeholder (with alt text) render correctly.
• External links verified (PyPI, GitHub repo, hosted docs, roadmap issue) return HTTP 200.
• Toolkit package has CI coverage (ruff, mypy, pytest with ~92% coverage) and a published wheel on PyPI; install command verified locally.
• CLI and agent examples were exercised against a live Notion workspace to confirm the documented flows.

Security & permissions

• Documentation lists required Notion scopes (pages:read, pages:write, databases:read, databases:write) and explains block whitelisting + payload size limits.
• Tokens are redacted in logs; Notion API failures surface as ToolException with status + error codes so teams can add approval steps.

Limitations & roadmap

• Calls out unsupported block types (gallery, board, timeline, database schema changes), attachment support, multi-workspace routing, and rate-limit caveats, with a link to the public roadmap issue in the external repo.

This PR touches documentation only. It gives reviewers a clear map of what’s ready, what still needs contributions, and how the community benefits.

[ccurme]

Thanks for this! Docs have moved to https://github.com/langchain-ai/docs

See contributing guidelines.