Improve CONTRIBUTING doc

[marcusdacoregio]

Some users are having a hard time configuring their environment to work with the Spring Security codebase.

We should review and improve the CONTRIBUTING doc in order to avoid contributors wasting time with stuff like formatting, imports, etc.

See:

• https://github.com/spring-projects/spring-security/issues/11073#issuecomment-1103116291

[marcusdacoregio]

The import order is not fixed automatically, we should provide the IDE configuration or at least mention what is the expected import order:

import java.*
<blank line>
import javax.*
<blank line>
import jakarta.*
import all other imports
<blank line>
import org.springframework.*
<blank line>
import static all other imports

[jzheaux]

I think setting up the dev environment should go in a separate document. The reason is that it branches out based on your IDE and perhaps your OS. I think Spring Framework's Build from Source document could be a guide.

[Crain-32]

As I just experienced the horrors of getting the formatting correct, I'd like to say it appears IntelliJ now supports the necessary import configurations.

For development environments Spring as a whole could provide simple dev containers or devfiles with the expected tools (Gradle, JVM Versions, etc) to contribute. (I will admit I haven't reviewed if other issues have covered this topic previously)