Update README performance benchmark

[suguanYang]

Summary

• replace the README benchmark screenshot with the updated five-series landing chart
• update benchmark copy to include Markitdown and avoid broad efficiency claims Markitdown does not support

Verification

• git diff --check
• screenshot generated from knowhere-dashboard PR Parsed result contract #45 local landing page

[suguanYang]

Updated the benchmark screenshot to keep the original README image resolution: . Verification: docs/assets/benchmark.png: PNG image data, 2928 x 1446, 8-bit/color RGB, non-interlaced, 2928 1446, and .

[suguanYang]

Correction/update: I regenerated and pushed the benchmark screenshot at the original README image resolution: 2928x1446.

Verification:

• file docs/assets/benchmark.png
• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check

[suguanYang]

Updated the README benchmark screenshot again from the latest landing chart:

• keeps the original image resolution: 2928x1446
• includes the Unstructured logo in the chart legend
• restores the original Efficiency bullet text per request

Verification:

• file docs/assets/benchmark.png
• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check

[suguanYang]

Updated the README benchmark screenshot after bolding Markitdown and Unstructured in the chart legend. The image still keeps the original 2928x1446 resolution.

Verification:

• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check

[suguanYang]

Follow-up screenshot pushed:

• Regenerated docs/assets/benchmark.png from the updated landing chart with Markitdown as the last benchmark series.
• Kept the screenshot at the original README resolution: 2928x1446.
• Markitdown/Unstructured labels use medium weight, not semibold.

Verification:

• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check HEAD~1..HEAD

[suguanYang]

Follow-up screenshot pushed:

• Regenerated docs/assets/benchmark.png with the new non-overlapping vertical value labels.
• Screenshot remains at the original README resolution: 2928x1446.

Verification:

• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check HEAD~1..HEAD

[suguanYang]

Follow-up screenshot pushed:

• Regenerated docs/assets/benchmark.png with the gentler sloped value labels.
• Screenshot remains at the original README resolution: 2928x1446.

Verification:

• identify -format '%w %h' docs/assets/benchmark.png -> 2928 1446
• git diff --check HEAD~1..HEAD