feature symfony#8592 [Flex] Update routing & controller chapters (weaverryan)

This PR was squashed before being merged into the 4.0 branch (closes symfony#8592).

Discussion
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[Flex] Update routing & controller chapters

More progress in updating all of the "Getting Started" guides. I will next do templating, configuration (though this has been done in symfony#8588) and also `service_container`, which I'd like to move into Getting Started.

Commits
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0cbbc28 fixing build errors
7f0163a tweaks
10563a9 Proofing the controller chapters
44f2ff8 more work on getting started docs
218e2d4 WIP controller chapter
8b923e6 WIP controller changes
89d56ea Updating routing for Flex

Author: weaverryan