docs: reposition product positioning statement to after benchmark chart

[EricNGOntos]

Closes #84

What Changed

Moves the bold positioning statement 'We're not developing the next MinerU — we're building document memory infrastructure that agents can effectively consume.' from the very top of the 'What We Are' section to immediately after the benchmark performance chart, as a blockquote before 'Key Advantages'.

Rationale

At the top of the page, the statement reads as a defensive opener before the reader has any context or evidence. After the benchmark chart — where the reader has just seen concrete performance data against raw parsing baselines — the same statement becomes a natural, confident interpretation of what those numbers mean: Knowhere is not competing on raw parsing fidelity, it is providing a higher-level memory infrastructure layer that agents can navigate and cite.

The flow now reads:

1. What Knowhere does (intro)
2. How it works (parse → retrieve)
3. Benchmark data (proof)
4. Positioning insight (why this matters)
5. Key Advantages (specifics)

Changes

• Removed bold statement from 'What We Are' opening
• Inserted as a > blockquote between the benchmark chart and 'Key Advantages'