Render AsciiDoc admonitions with the same style as Markdown alerts (NOTE/TIP/IMPORTANT/WARNING/CAUTION)

When an AsciiDoc file (e.g. `README.adoc`) is rendered by `github/markup`, admonition blocks (`NOTE`, `TIP`, `IMPORTANT`, `WARNING`, `CAUTION`) produce Asciidoctor's default HTML5 markup, which is a bare, unstyled ``:

```asciidoc
NOTE: This is a note.
```

renders as:

```html

This is a note.

```

Since github.com doesn't load Asciidoctor's stylesheet, this shows up as a plain, borderless table cell with the word "Note", no color, no icon, no visual distinction from the surrounding paragraph.

Compare this to the equivalent **Markdown alert**:

```markdown
> [!NOTE]
> This is a note.
```

which GitHub renders as:

```html

```

This picks up GitHub's built-in `.markdown-alert*` CSS automatically (colored left border, icon, colored title). AsciiDoc's five admonition types map 1:1 onto the five Markdown alert types (NOTE, TIP, IMPORTANT, WARNING, CAUTION), so there's no semantic gap, just a rendering gap.

## Proposal

`github/markup` already isolates the AsciiDoc rendering call in `lib/github/markups.rb`:

```ruby
Asciidoctor.convert(content, :safe => :secure, :attributes => attributes)
```

Rather than changing anything in Asciidoctor itself, github/markup could register a small custom HTML5 converter (a subclass of the default `Html5Converter`) that overrides **only** `convert_admonition`, and pass it as the `:backend`/`:converter` for this call. The override would emit the same `markdown-alert markdown-alert-` / `markdown-alert-title` markup already used (and already styled) for Markdown alerts, instead of Asciidoctor's `admonitionblock` table:

```ruby
class GithubAdmonitionConverter < (Asciidoctor::Converter.for 'html5')
register_for 'html5'

def convert_admonition node
name = node.attr 'name' # note, tip, important, warning, caution
label = node.attr 'textlabel' # Note, Tip, Important, Warning, Caution
%(

)
end
end
```

(the appropriate inline SVG icon per type could be added to match the ones GitHub already uses for Markdown alerts)

This requires no change to Asciidoctor itself and no new CSS on GitHub's side. It just reuses the classes/styles GitHub already ships for Markdown alerts.

I'm happy to contribute this change (the converter + wiring it into the `.adoc` render path) as a PR if that's a direction you'd be open to. Just let me know!

Start in lib/github/markups.rb and inspect the Asciidoctor.convert call and the existing .adoc rendering path. Done means AsciiDoc NOTE, TIP, IMPORTANT, WARNING, and CAUTION blocks emit the markdown-alert classes and title markup so GitHub's existing styling applies.

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