Add the Azure Files change tracker how-to guide

[YasanPunch]

Split out of #617 per review feedback.

Adds a how-to guide that derives created, modified, and deleted events from the Azure Files listener, using an application-side eTag snapshot plus a scheduled sweep to cover deletions (the listener reports presence, not change).

Depends on #617. The guide links to the Azure Files connector's setup guide and example pages, which that PR adds. Until it merges, the build reports two broken links to connectors/catalog/storage-file/azure.storage.files/ — onBrokenLinks is set to warn, so the build still passes, and both links resolve once #617 lands.

Every Ballerina snippet in the guide is compile-verified against ballerinax/azure.storage.files 1.0.0.

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