Update contribution guidelines

The current contribution guidelines are outdated, for example they still mention the use of `Nullable` instead of `Optional`. There are also still some files with wrong Javadoc formatting, that's why we should enable the Checkstyle rules to warn about that.

- Replaced contribution guidelines in `CONTRIBUTING.md` by a link to the SpongeDocs.
- Updated checkstyle and enabled the disabled rules to check for Javadoc formatting and some other smaller things.
- Rewritten Contribution Guidelines to match with changes like the use of `Optional` instead of `Nullable`, see SpongePowered/SpongeDocs#18.