[FLINK-6865] Update checkstyle documentation

[zentol]

This PR updates the checkstyle documentation.

• The plugin setup instructions now refer to strict-checkstyle.xml
• Added a section for importing the checkstyle in IntelliJ for adjusting the Code Style
• Updated the section regarding modules that don't use the strict checkstyle

[greghogan]

If the long-term plan is to have one checkstyle then we should keep the current plain name. We may be at the point where we can rename the checkstyles and update the in-progress modules to use a relaxed-checkstyle.xml.

I have not seen IntelliJ actually import settings from a checkstyle configuration. Not sure if this is user error or something is broken.

[zentol]

yes, it's a good idea to "flip" the checkstyle names.

I only checked a few things, but importing the checkstyle configuration, with the Checkstyle-IDEA plugin enabled, at the very least correctly configured the import order.

[zentol]

hmm....i just retried it and it didn't work this time. Let me investigate a bit.

[zentol]

Something the import did in fact set was spaces around operators. It may be that not all checkstyle rules are properly imported.

[zentol]

Only a (small) subset of checkstyle modules can be imported, which sadly doesn't include the ImportOrder module.

[greghogan]

Okay, second best may be to create an IntelliJ Code Style configuration for developers to import.

[zentol]

I don't know a lot about the intellij code style config; can we only define subset that is imported without affecting the rest?

Btw., I'm looking into contributing to the checkstyle plugin to add support for the rules that we need.

[zentol]

Here's a list of the currently supported modules btw.:

EmptyLineSeparator
FileTabCharacter
Indentation
LeftCurly
LineLength
NeedBraces
NoWhitespacesBefore
WhitespaceAfter
WhitespaceAround

[zentol]

Apart from the ImportOrder and AvoidStartImport modules everything that we could configure via IntelliJ Code Style seems to be covered.

[zentol]

@greghogan With the latest plugin version (5.6.1) the import layout is adjusted as well.

[zentol]

I've updated the PR:

• revert mentioning of strict checkstyle
• include note about not adjusted star import settings
• remove not about archetype modules as they are no longer valid

We should probably merge this after #4112 .