Description

This PR updates the Tailwind instructions to help with issue 6 in #3162:

When using custom pages or Starlight at a subpath and want to use Tailwind on both your main site/custom pages and Starlight, you propably want to use a default Tailwind setup for your main website/custom pages and a different one for Starlight which would use the Starlight’s Tailwind compatibility package.

1. My initial assessment was only focusing on the case of using Starlight at a subpath.

2. My initial idea was to add a small sentence in the existing Tailwind instructions, e.g.:

   If you are using Starlight at a subpath, you can use a different Tailwind CSS file for your main website often imported in a layout component.

3. After discussing the topic during Talking and Doc'ing, the idea of creating a entirely new sub-section "Add site-wide Tailwind support with Starlight at a subpath" was identified as a better approach. Even though it was vaguely similar to the existing "Add Tailwind to an existing project" section, it gives the user better instructions, and in the "Tailwind CSS" intro section, we can link to the various possible setups.

4. When working on the PR and checking various support threads, I realized that this setup may not only be useful for Starlight at a subpath, but also for users who want to use custom pages.

This PR keeps the new idea of creating a new sub-section but tries to cover both use cases: custom pages and Starlight at a subpath.

Other ideas discarded during T&D:

• Not re-using src/styles/global.css in our "Add Tailwind to an existing project" section but instead creating a new src/styles/starlight.css file so that it's more obvious that this file is only used for Starlight and the other one is only used for the main site/custom pages.

  This was discarded because src/styles/global.css is automatically scaffolded by Astro and this use-case is still a small percentage of users. That would mean that most people would end up with an src/styles/global.css file that they don't need.