chore: remove PullMd Loader

[chigwell]

Overview

This PR removes the PullMd integration (PullMd Loader + provider references) from the LangChain documentation, as the pull.md service was switched off on 25/12/2025. All related mentions, links, and navigation entries have been removed/updated to avoid sending users to a non-functional service.

Type of change

Type: Remove outdated content

Related issues/PRs

• GitHub issue: N/A
• Feature PR: N/A

• Linear issue: N/A
• Slack thread: N/A

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

• Documentation-only change (no library/runtime behaviour changes).
• Performed a repo-wide cleanup of PullMd references to prevent broken links and dead-end docs pages.