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
remove one of two empty_data includes
some language tweaks (AE, third-person perspective)
Fix YAML syntax highlight + remove trailing whitespace
Minimize horizontal scrolling in code blocks (improve readability)
Merge branch '2.2' into 2.3

Conflicts:
	book/security.rst
	cookbook/security/entity_provider.rst
Merge branch '2.3' into 2.4
Merge branch '2.4'

Conflicts:
	book/security.rst
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
minor #3499 Fix YAML syntax highlight + remove trailing whitespace (ifdattic)

This PR was merged into the 2.4 branch.

Discussion
----------

Fix YAML syntax highlight + remove trailing whitespace

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets |

Commits
-------

0723078 Fix YAML syntax highlight + remove trailing whitespace
document the ability to configure more than one mailer at the same time
fix include directive so that the contents are really included
replace Akamaï with Akamai
minor #3451 some language tweaks (AE, third-person perspective) (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

some language tweaks (AE, third-person perspective)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

0aad5d3 some language tweaks (AE, third-person perspective)
correct overriden option of button and add checkbox overriden option
correct overriden option of button and add checkbox overriden option
document overriden options of checkbox
document overriden options of choice field
Some additional tweaks and probably some merge conflict fixes
bug #3456 remove duplicate label (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
----------

remove duplicate label

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

Commits
-------

b64d73f remove duplicate label