docs: Rename Atlas Vector Search to MongoDB Vector Search and fix code examples

[jvincent-mongodb]

What this PR does / why we need it:

Renames "Atlas Vector Search" to "MongoDB Vector Search" across the MongoDB integration docs, since vector search now works in community edition too (not just Atlas). Also fixes the retrieve_online_documents_v2() code example to use the correct FeatureStore API signature, removes a PR back-link, and removes redundant description bullets per review feedback on #6351.

Changes:

• docs/reference/data-sources/mongodb.md: Rename Atlas Vector Search → MongoDB Vector Search, remove PR feat: Add Vector Search support to MongoDBOnlineStore #6344 link, add # pragma: allowlist secret, fix retrieve_online_documents_v2() example to match actual API
• docs/reference/offline-stores/mongodb.md: Remove description bullet points (per reviewer feedback)

Which issue(s) this PR fixes:

Follows up on #6351 (review comments after merge)

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Testing is not required for this change

Misc

This is a docs-only change (no code changes). Addresses post-merge review feedback from @caseyclements on #6351.