Simplify and clarify routing in quick start; remove default route

[afilina]

Q	A
Documentation	yes
Bugfix	no
BC Break	no
New Feature	no
RFC	no
QA	no

Description

• The default route is no longer a thing.
• Remove redundant code example, since the example below already combines the route and controller.
• "Now that we have" filler sentence that also assumes the reader read everything that came before.
• Explained how routes work on the most basic level.
• Use comments in the code example to identify key parts and how they're used.
• Change controller name to hello-controller, because the previous name was similar to the class name. It could have caused mistakes for less attentive devs. It also helps to know that this is just an arbitrary string, not some class name.
• Give a summary of the execution flow at the end.
• Add a link to routing page.
• Drop the "Controller services" section. It reads too much like internals docs. In any case, it seemed unnecessary in the quick start.