Update track's practice exercise contribution to cover track only vs problem spec exercises

[BNAndras]

The contributing to practice exercises guide should be updated to cover the difference between Java track only exercises and ones that go through the problem spec.

• Java track ones won't have any spec to sync with, does not require PR on problem spec.
• Problem specs one should be discussed on the forum first.

Notes:

java/reference/contributing-to-practice-exercises.md

Line 8 in 766c84e

To add a completely new exercise you need to open a pull request to the [problem specifications repository][problem-specifications].

Potential problem-specs repo PRs are expected to go through a forum discussion first before maintainers invite the contributor to make a PR. Having them open PRs first is not standard procedure and will cause friction for maintainers and contributors.

[kahgoh]

Updated the issue to more generally cover updating the track's practice exercise contribution guide.