docs: README structure and layout pass

[rejojer]

Top-level sections use single-hash headings again, with ### subsections underneath, as the July README did.

• Usage Guide section after the Quickstart: (a) the SDK client in three collapsed steps, (b) integration with four agent frameworks, each fold carrying the one-call form, the explicit form, and a printed result.
• Benchmarks split into Running PageIndex locally (indexing cost and time, query cost and accuracy) and Leading accuracy on FinanceBench, the latter restored from the August 19 README.
• TL;DR pull quote under the framework diagram, with the claims bolded.
• Charts centered at 75% (FinanceBench chart at its original 70%); header row drops Discord and adds Blog; Updates lists the File System again and dates entries as [Aug '26].
• Smaller fixes: citations example as one plain string, the Flash entry names layout extraction, the Cloud example loses a doubled blank line, link text covers only the noun in "Ready to Try It?", no trailing whitespace in code blocks.

Claude-Session: https://claude.ai/code/session_01BtbxfFP1wdkFou1FMCMA6k