HACKING/design: turn the remaining parts of crypto into a certs doc

doc/HACKING/design/01d-crypto.md → src/lib/crypt_ops/certs.dox

+/**
 
-
-### Certificates ###
+@page certificates Certificates in Tor.
 
 We have, alas, several certificate types in Tor.
 
@@ -9,6 +9,8 @@ won't explain X.509 to you -- possibly, no document can. (OTOH, Peter
 Gutmann's "x.509 style guide", though severely dated, does a good job of
 explaining how awful x.509 can be.)  Do not introduce any new usages of
 X.509. Right now we only use it in places where TLS forces us to do so.
+See x509.c for more information about using this type.
+
 
 The authority_cert_t type is used only for directory authority keys. It
 has a medium-term signing key (which the authorities actually keep
@@ -19,9 +21,12 @@ certificate.
 For new places where you need a certificate, consider tor_cert_t: it
 represents a typed and dated _something_ signed by an Ed25519 key.  The
 format is described in tor-spec. Unlike x.509, you can write it on a
-napkin.
+napkin.  The torcert.c file is used for manipulating these certificates and
+their associated keys.
 
 (Additionally, the Tor directory design uses a fairly wide variety of
 documents that include keys and which are signed by keys. You can
 consider these documents to be an additional kind of certificate if you
 want.)
+
+**/

src/mainpage.dox

@@ -31,6 +31,8 @@ Tor repository.
 @subpage intro
 
 @subpage dataflow
+
+@subpage certificates
 **/
 
 /**