[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