[doc] Update 4

[IAmNotHanni]

Work in progress:

• Rebase onto main
• Improve GitHub repo main page
• Add reference page about error handling using exceptions
• Merge pages in engine-design section into one page
• Remove message about CMake UI problems
• Refine FAQ section
• Add links to new Vulkan material
• Add links to new modern C++ material
• Write down our standard for code documentation using comments in a reference page
• Explain why CPU/GPU multithreading is important in FAQ of docs
• Improve reference page about octree collision detection

[IAmNotHanni]

I moved the "reference" section under "development" to the top. I think the reference is too important to be that far down in the menu list:

Opinions?