Create feature & target stability tables

[swoods-nv]

Slang should document clearly in a table (or two tables) the maturity/stability level of:

• Each target language
• Each major feature (e.g. ray tracing, mesh shaders, autodiff) -- and if the feature is only available on some targets, which ones.

[bmillsNV]

main landing page for target specific documentation. hub for all the behaviors that are target specific.