2 spaces

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.

Formats:
- Restructured Text
- JSON
- MarkDown

Specific Files:
- .ecrc
- CMakeLists.txt

Ignore:
- /patches/ directory, we will not be editing foreign patches for their
  indentation

Resolves #1929