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
Update Callback.rst

add square brackets to yaml codeblock (constraints: - Callback: [validate]). Otherwise an error "Expected argument of type array, string given" is thrown
[Components][HttpKernel] Add FINISH_REQUEST to events table
Updated install page to v2.4.*
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
Change symfony version on composer.json
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).
some tweaks to the Expression Language cache chapter
Update controller.rst

Invalid flash example for php template leading to::

FatalErrorException: Error: Call to undefined method Symfony\Bundle\FrameworkBundle\Templating\Helper\SessionHelper::getFlashBag()