Doc migration

[darrylabbate]

Currently, docs reside in riff-lang/site. Core documentation should either reside in this repo (/doc) or its own repo with a submodule pointer. I like the idea of keeping docs in their own repo for organization purposes, but keeping them in this repo makes it easier to update docs as features are changed/added.

• [x] Migrate old docs over
• [x] Update docs (last updated for 0.3.6)
• [x] Restructure and/or further split markdown files for better organization
• [x] Restructure files s.t. a man page could be generated from one or more markdown files (#19)
• [x] Document (either in README.md or doc/README.md) that the markdown files are written in Pandoc-flavored markdown - likewise that building docs depends on pandoc
• [x] Figure out what other companion files should be migrated over (if any)
• [ ] Write the EBNF grammar in a separate file (or parse from source code comments?)
  • [ ] Figure out a way to inline individual grammar parts in the markdown files without hardcoding.
• [ ] Update post-receive hooks on the server to reflect new doc structure
• [x] Add (minimum) Makefile recipe(s) for building docs
• [x] Convert docs to utilize implicit_header_references extension, as well as explicit link references (--reference-links) - which should clean up the docs quite a bit
• [x] Add defaults.yaml
• [x] Add secondary license for docs (CC-BY-4.0)

[darrylabbate]

Figure out what other campanion files should be migrated over (if any)

Initial thought: going forward, the website and playground (among other future projects) should consume this repo as a submodule. So any files which make sense to be common should probably be included here, such as the KDE syntax file(s).