[code quality] new make target that combines style and quality targets (#7310)

* [code quality] merge style and quality targets

Any reason why we don't run `flake8` in `make style`? I find myself needing to run `make style` and `make quality` all the time, but I need the latter just for the last 2 checks. Since we have no control over the source code why bother with separating checking and fixing - let's just have one target that fixes and then performs the remaining checks, as we know the first two have been done already.

This PR suggests to merge the 2 targets into one efficient target.

I will edit the docs if this change resonates with the team.

* move checks into style, re-use target

* better name

* add fixup target

* document new target

CONTRIBUTING.md

@@ -170,13 +170,19 @@ Follow these steps to start contributing:
    $ make style
    ```
 
-   `transformers` also uses `flake8` to check for coding mistakes. Quality
+   `transformers` also uses `flake8` and a few custom scripts to check for coding mistakes. Quality
    control runs in CI, however you can also run the same checks with:
 
    ```bash
    $ make quality
    ```
 
+   You can do the automatic style corrections and code verifications that can't be automated in one go:
+
+   ```bash
+   $ make fixup
+   ```
+
    If you're modifying documents under `docs/source`, make sure to validate that
    they can still be built. This check also runs in CI. To run a local check
    make sure you have installed the documentation builder requirements, by