docs: add a high-level framework overview that's easy to find

[bajtos]

Quoting from #5718 (comment)

List More useful info in the root level menu?

inspired by #5113 (comment)
A comparison between ours and RxJS: I like how they unfold and display some core concepts(each in one word) when page loads, to give readers an impression of what the framework does without looking into any details.

(I don't find a good candidate in the root level menu to unfold: "behind the scene" has a long list, others don't list sub-sections in a certain order. But the items in "Inside a LoopBack Application" is close to what I am looking for. )

Here is a list of the most basic concepts I could think of that makes at least REST API developers understand what this framework offers:

App --> Model --> Datasource --> Repository --> Controller --> API Explorer --> Relation --> Security --> Deploy
(Just an example)

Maybe we can refactor "Inside a LoopBack Application" to display ^ in the menu?

I see how it can be useful to be able to get an impression of what the framework does at high level. However, I am not convinced though that the sidebar is the right place for this kind of information. IMO, we should provide such overview on one of the introductory pages, e.g. "Inside a LoopBack Application" you mentioned or perhaps on the landing page (see Overview).

Let's discuss this idea and find a good solution.

This is a follow-up to #5718, see also #5549.