feat(ci): add markdownlint-cli2 for consistent markdown formatting

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.

docs/README.md

@@ -7,11 +7,13 @@ This folder contains the documentation source files for Spec Kit, built using [D
 To build the documentation locally:
 
 1. Install DocFX:
+
    ```bash
    dotnet tool install -g docfx
    ```
 
 2. Build the documentation:
+
    ```bash
    cd docs
    docfx docfx.json --serve
@@ -31,4 +33,3 @@ To build the documentation locally:
 ## Deployment
 
 Documentation is automatically built and deployed to GitHub Pages when changes are pushed to the `main` branch. The workflow is defined in `.github/workflows/docs.yml`.
-