Add long-lived cache headers for _astro/ build artifacts

[Copilot]

Astro's content-hashed assets under _astro/ were served with no Cache-Control header, leaving caching behaviour undefined. Since filenames are immutable by construction (content hash in the name), they can be cached indefinitely.

Changes

• docs/angular/public/web.config and docs/xplat/public/web.config: added an IIS <location> block scoped to the _astro/ directory that injects the target header:

<location path="_astro">
    <system.webServer>
        <httpProtocol>
            <customHeaders>
                <add name="Cache-Control" value="public, max-age=31536000, immutable" />
            </customHeaders>
        </httpProtocol>
    </system.webServer>
</location>

Mirrors the public, max-age=31536000 already applied to Google Fonts and other long-lived assets, with immutable added to suppress revalidation requests entirely.