MICROBA-923 Add ADR on course certificate allowlist (#26923)

lms/djangoapps/certificates/docs/decisions/001-allowlist-requirements.rst

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+Course Certificate Allowlist Requirements
+=========================================
+
+Status
+------
+Accepted
+
+Background
+----------
+Users can earn a course certificate in a particular course run (the certificate
+is stored in the GeneratedCertificate model). If a user has not earned a certificate
+but the course staff would like them to have a certificate anyway, the user can
+be added to the certificate allowlist for the course run. The allowlist is currently
+stored in the CertificateWhitelist model, and was previously referred to as the
+certificate whitelist.
+
+Requirements
+------------
+Even if a user is on the allowlist for a given course run, they won't necessarily
+receive a course certificate in the *downloadable* state. In other words, the user
+won't necessarily have a course certificate available to them. To receive a
+downloadable allowlist course certificate, the following things must be true at
+the time the certificate is generated:
+
+* The user must be enrolled in the course
+* The user must have an approved, unexpired, ID verification
+* The user must be on the allowlist for the course run (see the CertificateWhitelist model)
+* The user must not have an invalidated certificate for the course run (see the CertificateInvalidation model)
+* Certificate generation must be enabled for the course run
+* Automatic certificate generation must be enabled
+
+Note: the above requirements were written for the allowlist, which assumes the
+CourseWaffleFlag *certificates_revamp.use_allowlist* has been enabled for the
+course run. If it has not been enabled, the prior logic will apply.