Consistent landing page on each section

[yulin-dong]

There are different landing pages on each section.

On Learn: it is https://docs.avax.network/intro
On Quickstart: it is https://docs.avax.network/quickstart
On Subnet: it is https://docs.avax.network/subnets/when-to-use-subnet-vs-c-chain

We should be consistent on the landing pages. I would propose to add a README in each section as in https://docs.avax.network/quickstart

[laviniat1996]

Thanks for the feedback. Will try to fix it this week 👍

[meaghanfitzgerald]

@laviniat1996 we should sync on this, I was planning to remove the Quickstart tab as part of the docs revamp all together, among other upgrades. Consistent landing pages should be added to other tabs but the layout of our docs is going to change drastically so it might be counter-productive to do so now

[meaghanfitzgerald]

I am planning to make a diagram this week of the current state of our docs and the desired future state so that everyone can reference.