minor #4113 tweaks to the new reverse proxy/load balancer chapter (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

tweaks to the new reverse proxy/load balancer chapter

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

Commits
-------

bbc2f11 fix code block
6943da8 fix a headline
12840b1 improve linking between proxy config sections
feature #4081 [Components][ClassLoader] documentation for the ClassMapGenerator class (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Components][ClassLoader] documentation for the ClassMapGenerator class

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#2471)
| Applies to    | all
| Fixed tickets | #1134

Commits
-------

2ebe6c3 documentation for the ClassMapGenerator class
[#4081] Tiny tweak
Changes foobar.net in example.com

As recommanded by the RFC 2606, example.com(/net/org) should be used
as they are reserved for this usage.
minor #4115 [Documentation] [Minor] Changes foobar.net in example.com (magnetik)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4115).

Discussion
----------

[Documentation] [Minor] Changes foobar.net in example.com

As recommanded by the RFC 2606, example.com(/net/org) should be used
as they are reserved for this usage.

Commits
-------

715c0ed Changes foobar.net in example.com
Merge branch '2.3' into 2.4

* 2.3:
  Changes foobar.net in example.com
  [#4081] Tiny tweak
  Update introduction.rst
  documentation for the ClassMapGenerator class
  fix code block
  fix a headline
  improve linking between proxy config sections
Merge branch '2.4' into 2.5

* 2.4:
  Changes foobar.net in example.com
  [#4081] Tiny tweak
  Update introduction.rst
  documentation for the ClassMapGenerator class
  fix code block
  fix a headline
  improve linking between proxy config sections
Merge branch '2.5'

* 2.5:
  Changes foobar.net in example.com
  [#4081] Tiny tweak
  Update introduction.rst
  documentation for the ClassMapGenerator class
  fix code block
  fix a headline
  improve linking between proxy config sections
add order of translation formats

The preferred order of the different translation formats in
configuration blocks is Xliff, YAML, PHP. Thus, this has to be
documented in the contributing section. Since the order being used
before was Xliff, PHP, YAML, existing examples had to be modified
accordingly.
[Translation] added method to expose collected message
Rewrote Extension & Configuration docs
Update deployment-tools.rst

Added"Check requirements as A  in Common Post-Deployment Tasks
describe the usage of the RegisterListenersPass
some minor improvements
Added link to JSFiddle example
Fixed typos
Fixed typos
Update api_key_authentication.rst - POST Method
Check for api_key in request
[Components] consistent & complete config examples

Make configuration examples in the components section consistent
throughout the different configuration formats. Also fixing several
syntax issues.