docs: replace flow svg plot

[jacobowitz]

replace the svg plot for the readme flow

[github-actions]

Latency summary

Current PR yields:

• 🐢🐢 index QPS at 860, delta to last 2 avg.: -30%
• 🐢🐢 query QPS at 35, delta to last 2 avg.: -9%
• 🐎🐎🐎🐎 avg flow time within 1.8563 seconds, delta to last 2 avg.: +43%
• 🐎🐎🐎🐎 import jina within 0.4816 seconds, delta to last 2 avg.: +26%

Breakdown

Version	Index QPS	Query QPS	Avg Flow Time (s)	Import Time (s)
current	860	35	1.8563	0.4816
2.7.0	1150	50	1.256	0.3535
2.6.4	1327	26	1.3366	0.4053

Backed by latency-tracking. Further commits will update this comment.