[Documentation] Syntax guide

Incomplete outline of this portion of the documentation:

* Block Styles
* Scopes
* Classes
* Attributes
- Boolean attributes
- Attributes with values
* Legal Selectors
* Referencing a block or block style.
* `@block` - Referencing a block
* Styling an element based on another block's attribute.
* block paths
* Exporting Blocks
* Debugging a block's interface
* Block Interfaces and Inheritance
* Style Compositions
* Resolving Block Conflicts
* Override Resolution
* Yield Resolution

No documentation file, test, or entry point is named. Locate the syntax-guide documentation and use the listed topics as the scope; done means each outlined section, including block styles, interfaces, compositions, and conflict resolution, is documented.

Written by the indexing model from the issue text.