Update documentation.
Created by: LSmyrnaios
Updates in "Optionals.md":
- Updated the suggested exereme-version in "exareme.yaml", important for the deployment to work.
- Added a note for the ability to use the "nano" editor when editing the ansible-vault file, for people finding it difficult to work with "vim", which is the default editor for ansible.
- Small text-layout improvements.
Other:
- Fix a broken-link in "InputRequirements.md".
- Update the README for splitting the test datasets.