docs: rename non-FAQ question headings to statement style

[kheiss-uwzoo]

Summary

• Rename non-FAQ question-style headings to statement-style titles per NVIDIA Template Library guidance (overview H1, root index H1, LanceDB section, MkDocs nav label).
• Update cross-link text to match the new titles; preserve existing heading anchors so deep links keep working.
• Leave FAQ question headings unchanged; do not change numbered TOC section labels.

Test plan

• Confirm overview, index, and LanceDB headings are statement-style in the rendered docs.
• Confirm FAQ page headings remain questions.
• Spot-check nav label for overview and a few cross-links from multimodal / audio-video / troubleshoot pages.
• Confirm anchors #what-is-nemo-retriever-library, #what-is-nvidia-nemo-retriever, and #why-lancedb still resolve.

[greptile-apps]

Greptile Summary

Renames non-FAQ question-style documentation headings to statement-style titles while preserving their existing anchors.

• Updates the NeMo Retriever overview and root index H1 headings.
• Renames the LanceDB section and its local table-of-contents label.
• Aligns navigation and cross-link text with the renamed headings.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
docs/docs/extraction/overview.md	Renames the library overview H1 and explicitly preserves its existing generated anchor.
docs/docs/index.md	Renames the root documentation H1 and explicitly preserves its existing generated anchor.
docs/docs/extraction/vdbs.md	Renames the LanceDB heading and TOC label while retaining the existing explicit anchor.
docs/mkdocs.yml	Updates the overview navigation label to match the renamed page heading.

_{Reviews (2): Last reviewed commit: "docs: capitalize Overview in statement-s..." | Re-trigger Greptile}

[kheiss-uwzoo]

Folded into #2391 (same heading renames applied on that branch). Closing as duplicate.