Preserve angle brackets when converting @param <T> to Markdown

[timtebeek]

JavadocToMarkdownDocComment was dropping the angle brackets on generic type parameters, converting @param <T> into /// @param T instead of /// @param <T>. This happened because the Javadoc parser models a type-parameter name as a J.TypeParameter with the surrounding </> stripped, and printJ rendered only the inner name. The fix detects a J.TypeParameter name in convertParameter and re-adds the angle brackets. A genericTypeParameter test covering both a type parameter and a regular parameter is included and passes.