build-sys: Require explicit make schema-update to regenerate

Alternative to #1306

My recent change to use make for this seems to have introduced
a race condition mainly in Prow; perhaps something is not
preserving timestamps.

Files stored in git should not be touched
via a default make, as that is both subject to timestamp
problems and also creates friction.

So this change now requires an explicit manual generation; and
we also consistently add the generated doc file to git too.

.gitignore

@@ -8,4 +8,3 @@ maipo/
 *.pyo
 *.shellchecked
 .coverage
-mantle/cosa/schema_doc.go

mantle/Makefile

@@ -4,10 +4,10 @@ DESTDIR ?=
 ARCH:=$(shell uname -m)
 
 .PHONY: build test vendor clean
-build: cosa/schema_doc.go
+build:
 	./build cmd/*
 
-cosa/schema_doc.go: ../src/schema/v1.json Makefile
+schema-update:
 	./build schema
 
 .PHONY: install