Updated user-guide with Rayleigh Benard convection examples

Issue/feature addressed

An example of an active scalar coupled with IncNavierStokeSolver is unavailable in the user guide.

Proposed solution

Added a section that describes the setup for active scalar (Rayleigh Benard convection) in section 11.8

Implementation

See pages 230 - 231. user-guide.pdf

Tests

Suggested reviewers

@ssherw

Notes

Checklist

• User guide/documentation is updated.

• Changelog is updated.

  - [ ] Functions and classes, or changes to them, are documented.

  - [ ] 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).