cli, docs: Add a docs command for doc generation

This took a lot longer than it looks to get right. It's not perfect, but
it now reliably generates documentation which we can put into gohugo.

lang/interfaces/func.go

@@ -34,6 +34,7 @@ import (
 	"fmt"
 	"strings"
 
+	docsUtil "github.com/purpleidea/mgmt/docs/util"
 	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/engine/local"
 	"github.com/purpleidea/mgmt/lang/types"
@@ -279,6 +280,16 @@ type DataFunc interface {
 	SetData(*FuncData)
 }
 
+// MetadataFunc is a function that can return some extraneous information about
+// itself, which is usually used for documentation generation and so on.
+type MetadataFunc interface {
+	Func // implement everything in Func but add the additional requirements
+
+	// Metadata returns some metadata about the func. It can be called at
+	// any time, and doesn't require you run Init() or anything else first.
+	GetMetadata() *docsUtil.Metadata
+}
+
 // FuncEdge links an output vertex (value) to an input vertex with a named
 // argument.
 type FuncEdge struct {