Add pages on graphics debugging and profiling

[Calinou]

This documentation is aimed at new and returning rendering contributors (as well as users wishing to get more information than with Godot's own visual profiler). It aims to describe basic usage of GPU debugging/profiling tools on Godot projects.

TODO

• [ ] Document how to make use of rendering markers (such as Render Sky) in RenderDoc. Starting Godot with --verbose doesn't seem to suffice for Vulkan on Linux at least (although it works on Windows in NSight).
  • If you can figure it out, let me know.
• [ ] Document how to access shader sources in RenderDoc and NSight. Using --generate-spirv-debug-info currently doesn't work due to an engine bug.
• [ ] Document Radeon GPU Profiler in graphics profiling page.
• [ ] Document Radeon Memory Visualizer in graphics profiling page.