Fixed YAML code blocks in the book
Fixed (mostly) yaml code blocks

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats
minor #3497 Fix highlighting (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix highlighting

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | partially https://github.com/symfony/symfony-docs/issues/1886

Commits
-------

7c0cf89 Fixed (mostly) yaml code blocks
2a43a97 Fixed YAML code blocks in the book