Update guidance to recommend `markdowncli-2` and to use the VSCode extension

[khiga8]

I think we will want to update the guidance in the README per @smockle revelation in https://github.com/github/accessibility/pull/1941 that the VSCode extension doesn't work with the current configuration recommendation.

We'll additionally want to include the config updates in https://github.com/github/accessibility/pull/2535.