docs(readme): add link to learning portal, disambiguate difference with docs

[alexcg1]

Based on analytics data, a major source of traffic for our docs is our repo. This PR adds the learning portal to the readme to hopefully drive similar engagement. It also explains what each property offers to make it clear to user.

[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1176, delta to last 2 avg.: -4%
• 🐢🐢 query QPS at 21, delta to last 2 avg.: -10%
• 🐢🐢 dam extend QPS at 32822, delta to last 2 avg.: -8%
• 🐢🐢 avg flow time within 1.5804 seconds, delta to last 2 avg.: +20%
• 😶 import jina within 0.4226 seconds, delta to last 2 avg.: -2%

Breakdown

Version	Index QPS	Query QPS	DAM Extend QPS	Avg Flow Time (s)	Import Time (s)
current	1176	21	32822	1.5804	0.4226
2.6.4	1276	25	38630	1.1743	0.4559
2.6.3	1178	21	33282	1.4512	0.4089

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