Docs: Add & improve community documentation

[scottgerring]

What were you searching in the docs?

We should add a maintainers and a contributors guide. We can use the Python ones for reference.

Is this related to an existing documentation section?

No response

How can we improve?

Add the missing section

Got a suggestion in mind?

No response

Acknowledgment

• I understand the final update might be different from my proposed suggestion, or refused.

[scottgerring]

Starting work on this from https://github.com/aws-powertools/powertools-lambda-java/tree/doc-improvements

[scottgerring]

Maintainers guide is done. Contributors guide outstanding.