NMS-7219: Documentation for reviewer should follow new branch model

- Rearranged screenshots
- Divided section for working on local workstation vs. working in GitHub
- Added description to handle reviews with changes directly and through comments

JIRA: http://issues.opennms.org/browse/NMS-7219

Todo Review:
- [ ] Typo and grammar
- [ ] Formatting and conventions
- [ ] Content