docs: cron STAC validate job

workflows/cron/README.md

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+# Contents:
+
+- [cron STAC validate](#cron-STAC-validate)
+
+# cron STAC validate
+
+Workflow that validates the STAC metadata using [`stac-validate`](https://teams.microsoft.com/v2/?meetingjoin=true#/l/meetup-join/19:meeting_MDc1MWEzNzYtYTI4Yy00OWZmLWJhMzUtYjA1ZmU1ODBmNTg5@thread.v2/0?context=%7b%22Tid%22%3a%222134e961-7e38-4c34-a22b-10da5466b725%22%2c%22Oid%22%3a%2263d2d811-1d35-49f7-b9a3-c60e9b9a9ed1%22%7d&anon=true&deeplinkId=1c8a1674-d597-4c2d-ab7e-6ed968f086b7) and verify that the [STAC links](https://github.com/radiantearth/stac-spec/blob/master/collection-spec/collection-spec.md#link-object) are valid (using their checksums).
+
+- schedule: every day at 5am