docs: add Modal provider setup

[devennavani]

Summary

• add Modal to the provider directory
• document creating a managed endpoint and combining its proxy token
• show the /connect and /models flow for discovered workspace endpoints

Context

Modal model discovery landed in #39066, but the provider directory is maintained separately and did not include setup instructions.

Testing

• prettier --check packages/web/src/content/docs/providers.mdx
• git diff --check

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