Examples still use md prefixes

[Lokomojo]

#7241 removed all md prefixes, however the documentation examples still have them:

e.g https://material.angular.io/components/button/examples

[klofi]

Same for the Getting started page https://material.angular.io/guide/getting-started

[andrewseguin]

The documentation site has not yet been updated with the latest guides and examples. The repo's md files have already been changed so the next push should resolve this.

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