[capjan]

What changed

• README: new "Why MDU" section with six key points (one tool to set up, plain Markdown, linting in CI, static SVG diagrams, PDF export, migration from Material for MkDocs) and a link to the comparison.
• Docs/OtherTools.md: new "How MDU compares" table covering MDU, Material for MkDocs, Docusaurus, Starlight, and DocFX (build runtime, linter, diagrams, PDF export, source format).
• CONTRIBUTING.md: how to report issues, development setup with the .NET SDK, Conventional Commits, and pull request guidelines.

Why

First step of making MDU easier to discover: visitors should quickly see what MDU offers and how to contribute. The wording describes what MDU includes instead of setting it against other platforms. The repository description and topics on GitHub were updated separately.

For reviewers

• The MDU column was checked against the docs. The other columns are from general knowledge; please double-check the "Plugins" cells and Starlight's Mermaid support.
• mdu render Docs and mdu lint Docs pass; the #how-mdu-compares anchor used by the README link exists.