feat: add blacken docs to precommit

[samsja]

Automatically calling blacken docs in pre-commit hook

[github-actions]

Latency summary

Current PR yields:

• 🐢🐢 index QPS at 1024, delta to last 2 avg.: -19%
• 🐎🐎🐎🐎 query QPS at 46, delta to last 2 avg.: +19%
• 🐎🐎🐎🐎 avg flow time within 1.3136 seconds, delta to last 2 avg.: +12%
• 😶 import jina within 0.4117 seconds, delta to last 2 avg.: +5%

Breakdown

Version	Index QPS	Query QPS	Avg Flow Time (s)	Import Time (s)
current	1024	46	1.3136	0.4117
2.7.0	1021	47	1.2808	0.417
2.6.4	1515	29	1.0458	0.363

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