Restructure schema / docs folder

[fwkoch]

• Moved docs and examples to separate folder away from schemas.
• Renamed and split apart many of the schema files to be more specific

New structure looks like this:

schema/
|- myst.schema.json
|- commonmark/
|  |- commonmark.schema.json
|  |- commonmark.md  // docs
|  |- commonmark.yml  // examples
|- unist/
|  |- unist.schema.json
|  |- unist.md
|  |- unist.yml
|- etc
...

to

schema/
|- myst.schema.json
|- commonmark.schema.json
|- unist.schema.json
|- etc // json schemas
docs/
|- commonmark.md
|- ... // doc pages
|- examples/
|  |- commonmark.yml
|  |- etc // example files