[WIP] reorganize docs/

[tangent-vector]

This change attempts to slot the existing files under docs/ into a hierarchy that is more suitable for subsequent additions and improvements.

The core idea is to define a set of directories under docs/ that more clearly indicate where particular kinds of documentation can be found (and should go, when writing new docs):

• compiler/ : docs related to the Slang compiler

• compiler/command-line/ : docs related to slangc

• compiler/implementation/ : contributor-facing docs on parts of the compiler implementation.

• contributor/ : general contributor-facing docs

• gfx/ : docs about gfx

• gfx/guide : the gfx user's guide

• gpu-feature/ : docs on specific GPU API features and how they are exposed and implemented.

• historical/ : docs that are no longer relevant, but might not be good to delete entirely

• language/ : docs related to the Slang language

• language/guide/ : the Slang language user's guide

• language/reference/ : the in-progress language reference

• proposals/ : place for language/API/etc. proposals that should go through a more involved design/review process

• stdlib/ : docs about the types/functions/etc. exposed by the Slang standard library

• target/ : docs pertaining to specific targets, and feature support across targets

One obvious missing piece here is:

• compiler/api/ : docs (guide and reference) for using the Slang compiler via its API

[csyonghe]

We must update all references from main README and other docs e.g. build/conttribution guide and from shader-slang.org before merging this change.