Tidy up inc ns user-guide

Issue/feature addressed

Tidying up the user guide for the IncNS, making the synopsis consistent and re-structuring and modifying the simulation set-up. The work is not complete yet and can be further improved. But it is at a stage that can be independently merged and further revisions could follows in another MR.

Tests

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• [ ] Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• [ ] No extraneous files have been added (e.g. compiler output or test data files).