Add automated markdown linting to ensure consistent formatting across
all markdown files in the repository.
Changes:
- Add .markdownlint-cli2.jsonc configuration
- Create .github/workflows/lint.yml for CI/CD integration
- Fix all 908 existing markdown errors across 27 files
- Enforce ATX-style headings and asterisk emphasis
- Set consistent 2-space list indentation
This prevents markdown errors after project initialization and
maintains high documentation quality standards.
## Problem
The links to the contributing and support guides at the bottom of [the docs site home page](https://github.github.com/spec-kit/index.html) currently take you to a 404 page.
## Solution
Update the links to the correct URLs, pointing to the guides that are currently in the repo's root directory rather than within the `docs` folder.
