chore(website): added guidelines for LLMs

[lud-wj]

Summary

Add agent guidelines to the llms.txt output so that AI coding assistants using our documentation follow Welcome UI conventions:

• Theme tokens over arbitrary values: explains the --*: initial reset, lists the spacing scale with real pixel values, and links to the full theme reference.
• CSS custom property syntax: documents the px-(--spacing-xl) / shadow-(--elevation-20) pattern for using theme tokens as Tailwind arbitrary values.
• Drawer/Modal composition: Explain that Drawer and Modal already integrate Window sub-components (no need to import Window separately).

Also fixes a pre-existing bug where theme variables were missing from the https://welcome-ui.com/llms/foundations/theming/basics.md page (though it worked on my machine !!). Webpack resolved variables.js instead of variables.ts, bypassing the use-client-loader exclude and turning themeVariables into an empty client-reference proxy on the server.

Test plan

• Visit /llms.txt and verify the ## Guidelines section appears between Foundations and Components
• Visit /llms/foundations/theming/basics.md and verify color/spacing/radius tokens are listed (no more "No color tokens found" messages)

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