docs: splitted index

[Nayte91]

Q	A
Bug fix?	no
New feature?	no
Issues	no
License	MIT

Hello, Draft of some documentation's improvements (WIP):

1. Splitted "Form" chapter into new page. --> This one is pretty straightforward, goal is just to shrink the index size with a quick win; Form is a big and quite stand alone chapter.
2. Splitted "Communication Between Components: Emitting Events" and "Nested Components" in to a new "Communication between components" page. --> The goal is to regroup parts about how components can interact with each others. "Communication between components" can be renamed, but seems good.
3. Moved the orphan's "Dispatching Browser/JavaScript Events" into the "Working with the Component in JavaScript". --> It was after the moved "Communication" chapter, so it ended up alone. It belongs in either the "Working with the Component in JavaScript", or "Advanced functionality" part.

More to come in next commits:

• pass the tests,
• more splits if needed,
• look at twig/live component's splits,
• a chapter about emitting event in PHP, instead of going out in JS/going in in PHP,
• modify the angle of the documentation from "exhaustive list of how it works" to "how to do THIS or THAT",
• more examples, notes and warnings blocks in doc,

Comments are welcome!

[javiereguiluz]

I'm not judging this proposal, but as a general rule: most Symfony Docs readers strongly dislike having to switch between multiple articles to fully learn something. They overwhelmingly prefer a single, comprehensive article that contains all the information, even if it's long.