doc: add documentation for CRL generation (#7980)

Part of #7094

docs/CRLS.md

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+# CRLs
+
+For each issuer certificate, Boulder generates several sharded CRLs.
+The responsibility is shared across these components:
+
+ - crl-updater
+ - sa
+ - ca
+ - crl-storer
+
+The crl-updater starts the process: for each shard of each issuer,
+it requests revoked certificate information from the SA. It sends
+that information to the CA for signing, and receives back a signed
+CRL. It sends the signed CRL to the crl-storer for upload to an
+S3-compatible data store.
+
+The crl-storer uploads the CRLs to the filename `<issuerID>/<shard>.crl`,
+where `issuerID` is an integer that uniquely identifies the Subject of
+the issuer certificate (based on hashing the Subject's encoded bytes).
+
+There's one more component that's not in this repository: an HTTP server
+to serve objects from the S3-compatible data store. For Let's Encrypt, this
+role is served by a CDN. Note that the CA must be carefully configured so
+that the CRLBaseURL for each issuer matches the publicly accessible URL
+where that issuer's CRLs will be served.
+
+## Shard assignment
+
+Certificates are assigned to shards two ways: temporally and explicitly.
+Temporal shard assignment places certificates into shards based on their
+notAfter. Explicit shard assignment places certificates into shards based
+on the (random) low bytes of their serial numbers. All certificates
+implicitly have a temporal shard. Only certificates with the
+CRLDistributionPoints extension are considered to have an explicit shard.
+
+As of Jan 2025, we are planning to start assigning explicit shards at
+issuance time and then, after a transition period, turn off temporal sharding.
+
+## Storage
+
+When a certificate is revoked, its status in the `certificateStatus` table is
+always updated. If that certificate has an explicit shard, an entry in the
+`revokedCertificates` table is also added or updated. Note: the certificateStatus
+table has an entry for every certificate, even unrevoked ones. The
+`revokedCertificates` table only has entries for revoked certificates.
+
+The SA exposes the two different types of recordkeeping in two different ways:
+`GetRevokedCerts` returns revoked certificates whose NotAfter dates fall
+within a requested range. This is used for temporal sharding.
+`GetRevokedCertsByShard` returns revoked certificates whose `shardIdx` matches
+the requested shard.
+
+For each shard, the crl-storer queries both methods. Typically a certificate
+will have a different temporal shard than its explicit shard, so for a
+transition period, revoked certs may show up in two different CRL shards.
+A fraction of certificates will have the same temporal shard as their explicit
+shard. To avoid including the same serial twice in the same sharded CRL, the
+crl-updater de-duplicates by serial number.
+
+## Enabling explicit sharding
+
+Explicit sharding is enabled at the CA by configuring each issuer with a number
+of CRL shards. This number must be the same across all issuers and must match
+the number of shards configured on the crl-updater.
+
+Once we've turned on explicit sharding, we can turn it back off. However, for
+the certificates we've already issued, we are still committed to serving their
+revocations in the CRL hosted at the URL embedded in those certificates.
+Fortunately, all of the revocation and storage elements that rely on explicit
+sharding are gated by the contents of the certificate being revoked (specifically,
+the presence of CRLDistributionPoints). So even if we turn off explicit sharding
+for new certificates, we will still do the right thing at revocation time and
+CRL generation time for any already existing certificates that have a
+CRLDistributionPoints extension.