[Contributing] Clarify the rebase operation

I just mentioned the fact that `push -f` is usually required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).
[Contributing] Fix single vs double backtick issues
[Contributing] Add PSR-1 and 2 links
[Contributing] Replace mentions of 2.2 for 2.3

As per WouterJ request: https://github.com/symfony/symfony-docs/pull/3421#issuecomment-31585092
[Contributing] Fix "When you finish" => "When you are finished"

As per @xabbuh suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8661716
[Contributing] Replace HEAD~3 by upstream/master

As per @cordoval suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8670868
bug #3421 [Contributing] Cleaning the "contributing patch" page a bit (lemoinem)

This PR was merged into the 2.4 branch.

Discussion
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[Contributing] Cleaning the "contributing patch" page a bit

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

I just mentioned the fact that `push -f` is required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).

I also worked my way through the all document to replace wrong single back-tick occurrences by double backticks.

Finally, I converted the mentions of PSR-1 and PSR-2 to links toward the respective PSR.

PS: as per @WouterJ request (https://github.com/symfony/symfony-docs/pull/3421#issuecomment-31585092), I also replaced mentions of 2.2 by 2.3

Commits
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c0de1cb [Contributing] Replace HEAD~3 by upstream/master
732758b [Contributing] Fix "When you finish" => "When you are finished"
b25ec96 [Contributing] Replace mentions of 2.2 for 2.3
70ed645 [Contributing] Add PSR-1 and 2 links
6796ba8 [Contributing] Fix single vs double backtick issues
b35e62b [Contributing] Clarify the rebase operation