Improve user manual

[remkop]

• start with == Introduction heading, remove Example heading? - gives better Google results
• link from GitHub README to http://picocli.info
• link from manual (first paragraph): With <<ANSI Colors and Styles,colors>>
• how it aims to let users run picocli-based applications without requiring ...
• 2.4.1. Default Separators last example should be "// below is invalid (picocli will not recognize the --file option)
• Rename 3. Type Conversion to Strongly Typed (also update link at end of section 2.2)
• 3.1. Built-in Types: enums are not mentioned
• 3.1. Built-in Types: make this a bullet-point list
• 3.2. Custom Converters - change 2nd example to this:

App app = new App();
CommandLine commandLine = new CommandLine(app)
        .registerConverter(Path.class,     s -> Paths.get(s))
        .registerConverter(Duration.class, s -> Duration.parse(s));

commandLine.parse("-d", "PT15M", "file1.txt");
assert app.path.equals(Paths.get("file1.txt"));
assert app.duration.equals(Duration.parse("PT15M"));

• 3.4. Numeric Arguments:

new CommandLine(obj)
        .registerConverter(Byte.class,    s -> Byte::decode)
        .registerConverter(Byte.TYPE,     s -> Byte::decode)
        .registerConverter(Short.class,   s -> Short::decode)
        .registerConverter(Short.TYPE,    s -> Short::decode)
        .registerConverter(Integer.class, s -> Integer::decode)
        .registerConverter(Integer.TYPE,  s -> Integer::decode)
        .registerConverter(Long.class,    s -> Long::decode)
        .registerConverter(Long.TYPE,     s -> Long::decode);

• 11.1. Registering Subcommands

CommandLine commandLine = new CommandLine(new Git());
        .addCommand("status",   new GitStatus())
        .addCommand("commit",   new GitCommit();
        .addCommand("add",      new GitAdd())
        .addCommand("branch",   new GitBranch())
        .addCommand("checkout", new GitCheckout())
        .addCommand("clone",    new GitClone())
        .addCommand("diff",     new GitDiff())
        .addCommand("merge",    new GitMerge())
        .addCommand("push",     new GitPush())
        .addCommand("rebase",   new GitRebase())
        .addCommand("tag",      new GitTag());

• 13.1 Convenience API for Runnables - add clear headers BEFORE and AFTER
• Section 18 Download - Create separate section "Releases"