docs: improve the structure of "Behind the scenes" sidebar + find a better name

[bajtos]

At the moment, "Behind the scenes" contains a list of topics in a seemingly random order.

Let's find a better organization of the items in this list.

Ideas to consider:

• Sort the entries from A to Z, possibly keeping "Crafting LoopBack 4" at the top

• Reorganize the entries into subsection aligned around different usage scenarios or architecture layer they belong to: Build REST APIs, Access databases, Access existing services, Authentication/authorization, Cloud native, Dependency Injection & IoC, etc.

• Quoting [docs] Adopt four-quadrant documentation system #5718 (comment):

  Maybe we should use a consistent order from top to bottom, from left to right? For example:

  Application/Component/Context/Binding
  Server/Sequence/Action/Route/Controller/Repository/Service/Model/Relation/DataSource

We should update the contents of "Behind-the-scene.md" accordingly too.

See also:

• Improve the structure of "Usage scenarios" (how-to guides) sidebar docs: improve the structure of "Usage scenarios" (how-to guides) sidebar #5777

While we are discussing "Behind the scenes", let's try to find a better name for this section too (see the discussion in #5718 for more details).

This story is a part of #5113 Documentation improvements 2020Q3

Acceptance criteria

• categorize concepts into subsections by responsiblities
• reorganize entires in Behind-the-scene.md as well