[HTTP Cache] Validation model: Fix header name
[Security][Authentication] Fix instructions for creating custom password
encoders
Fixed a plural issue

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |
add versionadded directive for multiple option
[quick_tour] simplified drastically the first two sections of "The Big Picture" chapter
[quick_tour] simplified the "routing" section
[quick_tour] updated the "Controllers" section
[quick_tour] updated some screenshots
[quick_tour] finished the review of "The Big Picture" chapter
[quick_tour] updated "the view" chapter
[quick tour] simplified "the controller" chapter
[quick_tour] simplified "the architecture" chapter
[quick_tour] second pass to the "big picture" chapter
[quick tour] second pass to "the view" chapter
[quick_tour] second pass to "the controller" chapter and
added a section about displaying error pages
[quick_tour] second pass to "the architecture" chapter
[quick_tour] replaced "chapter" by "part" in some tutorial parts
[quick_tour] removed inline links
Bundle names should not be placed in literals
Removed a wrongly inserted comma