docs: add text server-side only.

[wootsbot]

Summary of the Change

This pull request updates the description of Flags SDK to clarify that it works exclusively on the server side. Previously, the documentation could be misinterpreted, leading users to believe the SDK could also run on the client side.

Why This Change?

• Clarity for Developers: By emphasizing that Flags SDK is server-side only, we prevent confusion in Next.js and SvelteKit setups.
• Fewer Errors: Without this clarification, users might attempt to use the library client-side, causing unexpected failures or behavior.

What Changed?

• The phrase "exclusively on the server side" was added to the main documentation sentence describing the SDK’s scope.
• No other sections were modified to keep the changes minimal.

Verification and Review

• This change only affects documentation, so there is no impact on functionality or risk introduced.
• The text was reviewed for consistency and spelling before submitting this pull request.

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