[doc] Update and improve documentation, part 4

[IAmNotHanni]

Work in progress:

Closes #553
Closes #484
Closes #520
Closes #388

• 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
• Refine FAQ section
• Fix source code view (integration of doxygen's output)
• Make docs front page consistent with vulkan-renderer repo's front page
• 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 documentation
• Move platform support into "Getting Started" and move "Getting Started" to the top
• Merge "Getting Started" with "Build Instructions"
• Remove CHANGELOG.rst to avoid duplication and just redirect to GitHub release page

Postponed:

• Invent a consistent terminology for "cube", "octree", "world", and others #561
• Explain why CPU/GPU multithreading is important in FAQ of docs #335