README: upgrade Public Preview note to GitHub admonition + status badge + link Azure Preview Terms

[xgerman]

What

Three small, README-only changes to make the project's Public Preview status more discoverable and legally sourced. No code, test, or behavior changes.

Diff

 # DocumentDB MCP Server

-> **Public preview.** This server is in public preview. Interfaces, configuration, and tool behavior may change without notice. Do not use it in production without your own review.
+[![Status: Public Preview](https://img.shields.io/badge/Status-Public%20Preview-orange?style=flat)](https://azure.microsoft.com/support/legal/preview-supplemental-terms/)
+
+> [!IMPORTANT]
+> **Public Preview.** This server is in Public Preview. APIs, configuration,
+> environment variables, and tool behavior may change in breaking ways before
+> General Availability. There is no SLA. Provided "as-is"; see the
+> [Azure Preview Supplemental Terms](https://azure.microsoft.com/support/legal/preview-supplemental-terms/).
+> Not recommended for production workloads without your own review.

Why each change

1. Add a Status: Public Preview badge under the H1

Shields.io badge linking to the Azure Preview Supplemental Terms. Matches the convention used by other Microsoft OSS repos in the same lifecycle stage and makes the project's pre-GA state visible without the reader scrolling past install buttons.

2. Upgrade the blockquote to a GitHub [!IMPORTANT] admonition

The existing line:

> **Public preview.** This server is in public preview. Interfaces, configuration, and tool behavior may change without notice. Do not use it in production without your own review.

…is a plain blockquote that renders as a thin gray bar with no color emphasis. Wrapping it as > [!IMPORTANT] makes GitHub.com render a colored callout that readers cannot miss; in any markdown renderer that doesn't support admonitions it degrades cleanly to a plain blockquote.

3. Link the canonical legal source

Microsoft Product Terms — Universal License Terms for Online Services — Previews — covers: "Previews are provided 'as-is', 'with all faults' and 'as available' and are excluded from the SLA and all limited warranties… Previews may not be covered by customer support."

That clause is the authoritative source for what "Public Preview" actually obliges (and doesn't oblige) Microsoft to. Linking the Azure Preview Supplemental Terms makes the README's preview claim sourced rather than asserted, so a reader can read the actual terms one click away.

Wording change (small)

Before	After
Interfaces, configuration, and tool behavior may change without notice	APIs, configuration, environment variables, and tool behavior may change in breaking ways before General Availability

Same intent, but:

• explicit about environment variables — the server's user-facing surface is substantially env-driven (AUTH_REQUIRED, TRUST_LOCAL_STDIO, ENABLE_*, CONNECTION_PROFILES, etc.), and env-var renames have already happened during preview (e.g., the ALLOW_UNAUTHENTICATED_STDIO → TRUST_LOCAL_STDIO rename in add default connection profile and stdio rate limit #83), so calling them out by name as a category that may break is honest;
• in breaking ways before General Availability is the canonical Azure preview-lifecycle phrasing and pairs cleanly with the supplemental-terms link.

Also added an explicit "There is no SLA" sentence — implicit in the supplemental terms but worth saying once at the top.

Scope

README-only; no source / test / behavior changes. git diff --stat:

 README.md | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com