[Reference][Form Types] Add missing "format" option to "datetime" field
Fix code block (minimise horizontal scrolling), typo in yaml
[Cookbook][Assetic] Fix "javascripts" tag name typo
Move note about "javascripts" and "stylesheets" blocks to separate note blocks
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
some language tweaks (AE, third-person perspective)
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
document the ability to configure more than one mailer at the same time
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)
[Reference][Form Types] Add missing options for "form" type
Improvements after review
Improve wording
Be more specific for the rendered field type
clarify that optional placeholders can only be used at the end of a route path
Merge pull request #3449 from bicpi/add_missing_form_options

[WIP][Reference][Form Types] Add missing options for "form" type
Merge pull request #3452 from xabbuh/issue-3450

clarify that optional placeholders can only be used at the end of a rout...
Merge pull request #3464 from xabbuh/issue-3440

document the ability to configure more than one mailer at the same time
Merge pull request #3466 from bicpi/add_missing_datetime_format_form_options

[Reference][Form Types] Add missing "format" option to "datetime" field
Update proxy_examples.rst

Code sample modified to reflect the actual usage