docs(readme): Litmus first-run callout and libreoffice typo

[kheiss-uwzoo]

Summary

• Docs-only follow-up from Litmus Docs Readiness + ::vdr on NRL: promote the LanceDB corpus-size warning before first ingest examples, point CLI first-run at ../data, and omit .vdb_upload() from the single-PDF inspect example so the happy path does not surprise new users at indexing.
• Fix libeoffice → libreoffice in the Office prerequisite prose (command was already correct).
• Align callout wording with current LanceDB behavior on main: very small corpora may skip the vector index or use fewer IVF partitions than the default (not an absolute hard-fail).
• No nemo_retriever/src/**, CLI, or default behavior changes.

Out of scope (intentionally)

• Performance Guide content (live page already 404 / excluded from MkDocs nav).
• Competitive positioning blurb (needs PM copy).
• Clearer CLI error on small-corpus index failure (optional eng PR).

Test plan

• Skim README Run the pipeline: corpus callout appears before first examples
• Confirm CLI examples use ../data and Python single-PDF example has no .vdb_upload()
• Confirm callout describes skip/clamp behavior (not absolute failure)
• Confirm libreoffice spelling in Office prerequisite sentence
• No runtime/code files in PR diff

[kheiss-uwzoo]

Superseded by #2430 — the Litmus README first-run guidance (LanceDB corpus-size / IVF callout, directory first-run, and the libeoffice -> libreoffice typo fix) is folded into the combined 26.08 RC readiness PR. Closing to consolidate the 26.08 docs changes into one review.