docs: add link to Jina 2 docs

[guenthermi]

This adds a link to the old Jina 2 documentation in the "Developer Reference" section. This might be useful for people to maintain and migrate old projects written for version 2.

[github-actions]

Latency summary

Current PR yields:

• 🐢🐢 index QPS at 887, delta to last 2 avg.: -21%
• 🐢🐢 query QPS at 35, delta to last 2 avg.: -29%
• 🐎🐎🐎🐎 avg flow time within 1.7514 seconds, delta to last 2 avg.: +28%
• 🐎🐎🐎🐎 import jina within 0.4528 seconds, delta to last 2 avg.: +31%

Breakdown

Version	Index QPS	Query QPS	Avg Flow Time (s)	Import Time (s)
current	887	35	1.7514	0.4528
3.0.2	1096	47	1.6109	0.3567
3.0.1	1165	52	1.1139	0.3317

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

[github-actions]

📝 Docs are deployed on https://docs-link-jina2-docs--jina-docs.netlify.app 🎉