docs: replace stale line-number citations in mongodb.md with named citations

[hasnaintypes]

Summary

• Replaces all 14 stale mongodb.ts:N / factory.ts:N line-number citations in docs/providers/mongodb.md with named citations (method name + bare file link), matching the convention established in mssql.md/trino.md/oracle.md.
• Adds docs/providers/mongodb.md to NAMED_CITATIONS in tests/unit/provider-docs-monitoring-citations.test.ts so the doc stays guarded against future line-number drift.

Fixes #543

[cevheri]

Thanks(five merged PRs - great), this is the mongodb.md half of the pair done the same way as #563.

Checked against the tree: the fourteen citations are the thirteen mongodb.ts:N from the issue plus the one factory.ts:88, no .ts:N is left in the file, and all ten names resolve to declarations.

The guard bites in both directions: putting one line number back fails the first case, and misspelling a listed method fails the second. CI is green end to end. Merging.