SKA1 simulation tools documentation added to Solution Intent

To continue the excellent work carried out in PI5 documenting OSKAR and RASCIL (https://sim-tools.readthedocs.io/en/latest/), these pages should be made more easily discoverable though the developer portal by means of a top level section (or alternative solution).

This ticket is a short time-boxed activity to perform an incremental update to this documentation, which should be extended to include a short curated list describing and linking to the key scripts, data sets and other information (requirements, estimated runtimes for typical hardware etc) needed to access or repeat a number of the SKA1 simulations that have been developed in previous PIs. The overall goal of this activity should also be to make it as easy as possible for new and existing SKA developers, as well as other external stakeholders, to discover, make use of, and contribute to SKA1 simulations, scripts, and tools in future PIs.

As this documentation will almost certainly need to be extended over time some (minor) refactoring of the top level page(s) should be considered to better accommodate new contributions to this space. 

Stretch: As the long term goal for simulation tools is to have an ecosystem of usable, interoperable tools for simulation of SKA data products, this ticket could also be used to extend the current documentation on RASCIL and OSKAR to describe and provide links to other simulation tools that can been used for SKA1 simulations. 

Stretch: As it is envisaged that simulation tools will be a core part of future SKA software development, it would also be useful to consider supporting the SKA architecture team in documenting (or linking documentation) describing simulation tools in the SKA solution intent repository (https://confluence.skatelescope.org/display/SWSI/Product%3A+Simulations)