[dub-reference/platform_specifications] Basic content

[WebFreak001]

• [x] Describe what platform specifications are / how platform definitions look
• [x] tell about partial platform specifications
• [x] link to build settings page for this + show basic usage / simple example
• [x] 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

[renatoathaydes]

Please update the above list as some items can be ticked off.

[WebFreak001]

thank you very much, documentation help is always very much appreciated!

[renatoathaydes]

I actually enjoy writing documentation :). As I learn more, I will keep contributing as possible.