Reflectometry: Document Script Library

As a developer, I would like to have documentation on the scripting library used for reflectometry experiments available on the dev wiki.

### Acceptance Criteria:
Developer wiki provides information on:
- Common reflectometry scripting methods
- Pre-requisites for running a reflectometry experiment via scripting. This should be at least detailed enough to get set up with a dev machine so that it can dry run scripts. e.g.
- expected blocks and naming conventions (slit gaps, beamline mode)
- How to define a sample
- How are instrument constants populated (pending https://github.com/ISISComputingGroup/IBEX/issues/4331)