Preserve Markdown frontmatter and check documentation sources

[timkpaine]

Preserve YAML frontmatter when formatting Markdown in all eight Python template variants by adding mdformat-frontmatter to the generated develop extra. This keeps MyST page configuration, such as heading_anchors, from being rewritten as Markdown headings.

Include existing docs/src directories in both formatting and spelling targets alongside README and optional wiki content. Use Make’s wildcard expansion so projects without that directory continue to lint and format successfully. Preserve Cython’s existing docs/development.md checks.

Generated all eight variants with docs/wiki enabled and disabled: 16 projects pass documentation checks with an absent source directory and a formatted frontmatter page. Deliberately malformed Markdown and misspellings fail lint, and fix-docs repairs them while preserving the YAML header. The generated pure-Python project also passes develop, lint, distribution-content checks, tests, and build.