encoding/protobuf: include annotations heading

This adds a section heading to the encoding/protobuf package
documentation for the set of annotations which CUE understands in
Protobuf options.

Adding this heading will allow us to link to this section on the
rendered page at https://pkg.go.dev/cuelang.org/go/encoding/protobuf
from
https://cuelang.org/docs/concept/how-cue-works-with-protocol-buffers/,
which is desirable so that that page doesn't need to explain every
annotation that CUE permits.

Signed-off-by: Jonathan Matthews <github@hello.jonathanmatthews.com>
Change-Id: I1e7f9c4ebaddb12961e5feae19e18742066353e4
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193847
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

encoding/protobuf/protobuf.go

@@ -64,6 +64,8 @@
 //	Timestamp      time.Time        See struct.proto.
 //	Duration       time.Duration    See struct.proto.
 //
+// # Annotations
+//
 // Protobuf definitions can be annotated with CUE constraints that are included
 // in the generated CUE:
 //