docs(astro): Add note about env docs

[s1gr1d]

DESCRIBE YOUR PR

Adding small notice about using environment variables in Astro.

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PRE-MERGE CHECKLIST

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• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 9, 2025 9:29am

2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
changelog	⬜️ Ignored (Inspect)	Visit Preview		May 9, 2025 9:29am
develop-docs	⬜️ Ignored (Inspect)	Visit Preview		May 9, 2025 9:29am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	11.16MB	-9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.48MB	-6 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-client-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
static/chunks/pages/_app-*.js	-3 bytes	868.7kB	-0.0%
static/chunks/8165-*.js	-3 bytes	410.1kB	-0.0%
static/bS6M2CMGcNHr2QpuH65VC/_buildManifest.js (New)	578 bytes	578 bytes	100.0% 🚀
static/bS6M2CMGcNHr2QpuH65VC/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/AfcpXtLjtpmc0LSqtb_SU/_buildManifest.js (Deleted)	-578 bytes	0 bytes	-100.0% 🗑️
static/AfcpXtLjtpmc0LSqtb_SU/_ssgManifest.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️

view changes for bundle: sentry-docs-server-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.75MB	-0.0%
../instrumentation.js	-3 bytes	1.08MB	-0.0%
9523.js	-3 bytes	1.05MB	-0.0%