docs: clarify that proxy/middleware matches static assets by default (#92441)
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### What?
Adds an explicit note about Proxy's default matching behavior at the top
of the **Matcher** section in the Proxy API reference — the first place
users look when configuring which paths Proxy runs on.
### Why?
Users have reported issues where Tailwind styles and other static assets
fail to load because their Proxy auth logic intercepts `_next/static`
requests and redirects them (see #92435). The existing docs mention that
Proxy runs on "every route" but don't explicitly call out static assets,
image optimizations, or public folder files — making this a common
pitfall that gets misdiagnosed as a CSS/PostCSS problem.
### How?
Added an inline paragraph at the top of the **Matcher** subsection
explaining that without a `matcher`, Proxy matches every request —
including `_next/static`, `_next/image`, and `public/` files — and that
this can break asset loading. Links to the existing Negative matching
section.
Fixes #92435
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Joseph <icyJoseph@users.noreply.github.com>