Improve README with better document structure and addition of sections with more high-level explanations.

[andre-senna]

An example of "use case" of the mentioned "high level" documentations is the first paragraph in the description of this issue: https://github.com/orgs/singnet/projects/6/views/1?pane=issue&itemId=84663712&issue=singnet%7Cdas-query-engine%7C355

The mentioned issue could have been created to someone that doesn't have any previous knowledge about how DAS Node is designed and how it works. Put yourself in the boots of such person and think about how you'd like to see the documentation structured and which information you'd like to read there.

Another "use-case" which should drive the same exercise is someone being assigned to this other issue: #48