docs: Add guide for integrating Django Tailwind into existing projects

[Copilot]

Users had no guidance on adding Django Tailwind to a project with pre-existing apps, scanning templates across multiple apps, or centralizing styles in a single theme app.

New page: docs/existing-project.md

• Single theme app covers everything — clarifies that one theme app is the global entry point; no changes needed to existing apps
• Source scanning — explains that the default @source "../../../**/*.{html,py,js}" already covers all apps in a standard layout
• Non-standard layouts — shows how to stack multiple @source directives when apps live under a subdirectory (e.g. apps/)
• Centralized styles — demonstrates using theme/static_src/src/styles.css as the single CSS entry point for custom variables and shared component classes
• Existing base template — shows how to drop {% tailwind_css %} into an existing base.html without touching the theme app's own template

Updated docs/index.md

Added link to the new page in the table of contents (between Installation and Standalone vs npm-based).

Tailwind CSS v3 is deprecated; all examples use v4 @source directives only.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Doc request: Adding django-tailwind to existing apps</issue_title>
<issue_description>From the documentation it's unclear how to add django-tailwind to a app that already exists and how to work with multiple apps.

I'd also appreciate some guidance on how to proceed if I want to keep a central entry point for tailwind. In some projects I prefer a global static_src - how would I set that up?</issue_description>

<agent_instructions>Try to tackle this issue. I wonder how smart you are, Copilot.
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Doc request: Adding django-tailwind to existing apps #243

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