docs: refine README headline, intro and TIP copy

[EricNGOntos]

Summary

Final polish pass on the README positioning section (follows PRs #70, #71, #72).

Changes

1. Updated H1 headline: Changed from "Prepare unstructured data for AI Agents" to "Build AI Agent Memory from Real-World Documents" to better reflect the product's core value proposition.
2. Condensed product description: Removed em-dashes, tightened wording while keeping all pipeline steps (parsing, hierarchy, multi-modal, graph).
3. Condensed TIP callout: Replaced verbose phrasing ("stands on the shoulders of", "Beyond parsing, Knowhere focuses on what turns raw extraction into...") with direct verb-driven list. Swapped "open-source" for "well-known" per user preference.
4. Updated bibtex citation title to match the new H1.
5. Updated slogan to specifically name MinerU as a contrast point per user direction.

Documentation-only. No code changes.