livehtml: Update --ignore pattern to use `**`

[joverlee521]

Resolves https://github.com/nextstrain/docs.nextstrain.org/issues/193

The double asterisk (**) glob expression worked for both sphinx-autobuild-2021.03.14 and sphinx-autobuild-2024.2.4. when I tested locally.

Checklist

• [x] Checks pass

[joverlee521]

Merging pre-review to improve my own dev cycle. Please chime in if there's a better way to do this!