docs(elm): simplify migrations help text and link to aka.ms/adoELM (#1518)

* docs(elm): simplify migrations help text and link to aka.ms/adoELM

Update the 'devops migrations' command group long-summary and doc/migrations.md to point users to https://aka.ms/adoELM instead of describing limited availability.

* docs(elm): wrap aka.ms/adoELM as markdown link to fix MD034 lint

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Co-authored-by: Demo User <demo@microsoft.com>