[dub-reference/platform_specifications] Basic content

[WebFreak001]

• Describe what platform specifications are / how platform definitions look
• tell about partial platform specifications
• link to build settings page for this + show basic usage / simple example
• list common build settings (just platforms, just architectures and how they can also be compiled)
• list full tables for platforms, architectures and compilers
• there is some sample code in dub-guide/recipe, which links to here - might want to extend on that code sample
• might want to look into if it's possible to auto-generate some tables here using the CI / a script in this repo, to keep things up-to-date