Consolidate docs and establish common categories

[ViktorHofer]

Copying requirements from https://github.com/dotnet/consolidation/blob/master/Documentation/directory-structure.md:

Proposed docs structure

  • docs
      ○ design
          § coreclr
	      - concepts (this is where botr content except Introduction to the Common Language Runtime (CLR) goes)
	      - ryujit (design documents specific to RyuJIT. RyuJIT is very large part of CoreCLR so it may deserve its own directory.)
	  § mono
	  § features (this is where the small scope design documents goes, e.g. https://github.com/dotnet/coreclr/blob/master/Documentation/design-docs/AssemblyLoadContext.ContextualReflection.md)
      ○ project
      ○ manpages
      ○ coding-guidelines
      ○ workflow
          § building
          § testing
          § profiling
          § performance

@dotnet/runtime-infrastructure