8796 update mac doc

[ric394]

Resolves #8796
Resolves #8756
Resolves #7253

• adds the required install docs for mac.
• updates the existing mac compile docs.

[par456]

Some notes to add:

• More on how to install homebrew (from console, and setup commands)
• Pointing out specifically what runtime is needed (x64)
• Cannot be amd64 .net library, ApsimNG for MacOS is built only for x64
• The command needed to put dotnet into path
• Using dotnet --version to check if dotnet is installed
• Must be admin (requires sudo to install parts)
• The fact that the the first time you open apsim it gives a security warning, and the solution is to close the warning and open it a 2nd time.