docs: use stable Teams CLI package

[heyitsaamir]

Summary

• Updates Teams setup docs to install @microsoft/teams.cli instead of the old @preview tag.

Why

• Teams CLI is stable now, so the docs should point folks at the stable package. Nice and tidy.

Interesting bits

• This touches both the public docs page and the adapter README so they don't disagree.

Test plan

• Not run; docs-only change.

[vercel]

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