[storage]Add a summary migrationguide for storage sdks.

[EmmaZhu]

Add a summary migration guide document to link to detailed migration guides for each separating storage SDKs.

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[ramya-rao-a]

This page is not a guide in itself, so we can cut the part that mentions that

Suggested change

	This guide is intended to assist in the migration to version 12 of `@azure/storage-blob`, `@azure/storage-file-share`, `@azure/storage-queue` and `@azure/data-tables` from the legacy `azure-storage` package. It will focus on side-by-side comparisons for similar operations between the two packages.
	Below are links to detailed migration guide for each separating package of different services:
	Below are links to detailed guides to migrate from the legacy `azure-storage` package to the latest versions of `@azure/storage-blob`, `@azure/storage-file-share`, `@azure/storage-queue` and `@azure/data-tables`.