[docs] Clarify where to install the license

[oliviertassinari]

This PR is trying to better answer https://mui.zendesk.com/agent/tickets/2851.

I'm also doing a few small polishes after #4298

1. Move License key installation from a h3 to a h2. I don't think that it should be under the legal license section. The license key is a different notion, something to help developers remember about the legal license. I feel that having it at the same level as the license section would make it clear.
2. Mention a bit how Premium is not available yet.
3. Start the license section to explain why it exists
4. Update the README to link more of the docs, less work to keep one place up to date than one.
5. Cover https://mui.zendesk.com/agent/tickets/2946 too. It's the same question as in https://stackoverflow.com/questions/70838326/where-should-the-mui-x-pro-license-key-be-installed/70901041#70901041

https://deploy-preview-4452--material-ui-x.netlify.app/x/advanced-components/

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	361	530.7	423.3	426.42	62.164
Sort 100k rows	ms	497.3	1,040.4	497.3	811.04	209.471
Select 100k rows	ms	177.1	291	197	214	39.775
Deselect 100k rows	ms	119.9	199.7	167.1	163.38	33.007

Generated by 🚫 dangerJS against 126d591

[joserodolfofreitas]

👌