Restructure project directories (#333)

`/client/web` -> `/frontend`
`/client/cli` -> *delete for now*
`/client/native` -> *delete for now*
`/core/editor` -> `/editor`
`/core/document` -> `/graphene`
`/core/renderer` -> `/charcoal`
`/core/proc-macro` -> `/proc-macros` *(now plural)*

docs/design/README.md

@@ -0,0 +1,8 @@
+# Graphite design notes
+
+Notes and details about the product design, workflow, user experience, and UI for the Graphite editor and systems. More to come as scattered outdated notes get cleaned up and moved here.
+
+- [Feature goals](feature-goals.md)
+	- [Possible use cases](feature-goals.md#possible-use-cases)
+	- [User stories](feature-goals.md#user-stories)
+- [Inputs and keybindings](inputs-and-keybindings.md)