Improvements to registering an extension (#2)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #9644 (symfony) and #3231 (symfony-docs)

1. The original text is good but the topic of that paragraph is not "extension conventions" but "registering an extension" (i.e. if you follow these conventions then register goes like so ..)

2. Cleaned up different terminology "Conventions" v "Standards" -> just conventions now

3. Move this part to a more logical place. First create then register, then modify for additional needs.
Changed due to comments in repo collab
Fix formatting
Remove confusing sample code
Improve fixes
[Components][OptionsResolver] Fix&improve replaceDefaults() description
[Components][Console] Fix askConfirmation() description
add API link
Fix wrong list indentation
Change link to download yuicompressor
Fix some bash and text code blocks
Incorrect code: useIncludePath()

Docs of v2.2 and later not in sync with code from v2.2 and later. See: https://github.com/symfony/ClassLoader/blob/2.2/ClassLoader.php#L108
terminate all but the last list items with a semicolon
Move curly bracket to be on the same line

This makes PSR coding standard to pass and also gives more consistency between examples (as curly brackets are on the same line).
Update controller.rst

Invalid flash example for php template leading to::

FatalErrorException: Error: Call to undefined method Symfony\Bundle\FrameworkBundle\Templating\Helper\SessionHelper::getFlashBag()
add note on return values when null is passed to a data transformer's transform() method
remove installation details for PHPUnit
improve the serialization of custom user models
Remove invalid tip for multiple providers

As confirmed by stof in #9734, the tip has never worked and is not going to work
[Cookbook][Dynamic Forms] Simplify first example