Add a formatting/style guide

It should include:

- Do not hard-wrap text in .rst files.
- Every sentence begins on a new line.
- Use headings in the order `***` (first in any file), `===`, `---`, `~~~`.
- Spell-check all added text.

The issue names .rst files but no target file; start by locating the documentation entry point where contributor formatting guidance belongs. Add the four listed rules there, including the heading order and spell-check requirement, and verify the guide is complete against the issue text.

Written by the indexing model from the issue text.