Expand README and add hero GIF recording guide

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Convert MetadataHelpProvider.GetRootHelp to generate markdown with
  ## headings, bullet lists, and backtick formatting
- Fix CliHost.WriteRootFlag to render via MarkdownRenderer.RenderToAnsi
  instead of plain stdout.WriteLine
- Fix HelpCommand.RunAsync to display markdown in a fullscreen
  Terminal.Gui Markdown viewer using RunnableWrapper
- Add tests for --help ANSI output, help --cat, and MetadataHelpProvider
  markdown generation

Fixes #5

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…on tests

The help command's TUI viewer rendered empty content because
Markdown.Text was set before the view was initialized/laid out.
Following the pattern from gui-cs/clet's MarkdownClet, set the text
inside the Initialized event handler on the containing Runnable.

Also switches from RunnableWrapper to a plain Runnable with an embedded
Markdown view — the help viewer is read-only and needs no result
extraction.

Adds integration tests using Application.Create()/Init(ansi) with
StopAfterFirstIteration to verify:
- The TUI renders without hanging
- Driver contents contain expected command text
- Subcommand help renders correctly
- RenderCatAsync produces ANSI output

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Help command and --help flag should render markdown

- Re-record docs/images/hero.gif using tuirec v0.4.0 with the example app
- Rewrite README for clarity: add badges, a Why section, feature table,
  real JSON output example, contributing pointer, and cleaner structure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add hero GIF and polish README