Port the site documentation from APT to Markdown

src/site/markdown/index.md

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+
+# Apache Maven Shared Utils
+
+This project replaces [plexus-utils](https://codehaus-plexus.github.io/plexus-utils/) for Maven.
+
+It is not a 100% API compatible replacement. We cleaned up methods, added generics, and removed unused code.
+
+This project adds new features, like [styled message API](./apidocs/org/apache/maven/shared/utils/logging/package-summary.html).
+
+# Why?
+
+plexus-utils consisted mostly of code that originated in various Apache projects. Maven Shared Utils is based on those original Apache-source implementations.
+
+# Why not commons?
+
+We prefer code to use commons-* where appropriate. The plexus-utils versions became incompatible with the Commons versions over time. Migrating is not always a 1:1 operation. Migrating to Maven Shared Utils works in most cases as a 1:1 operation.