feat(docsite): optional site_url and site_label link back to the main site

- site.yaml keys site_url and site_label, validated: http(s) URL with a host
  or a path starting with a single /; a label needs a URL
- docs:build and docs:serve flags --site-url and --site-label override them
  the way --base-url overrides base_url
- every page header, the 404 page included, links back with the explicit
  label, else the URL host, else Home; unset output is unchanged
- docs/console/utilities.md documents the keys and flags

cmd/summer/docs.go

@@ -29,6 +29,8 @@ func docsBuildCommand() bonfire.Command {
 			{Name: "src", Description: "Docs source directory (default <root>/docs)"},
 			{Name: "out", Description: "Output directory (default <root>/site)"},
 			{Name: "base-url", Description: "Base URL for site links (overrides site.yaml base_url)"},
+			{Name: "site-url", Description: "Main site URL linked from the docs header (overrides site.yaml site_url)"},
+			{Name: "site-label", Description: "Label of the main site link (overrides site.yaml site_label; default: the URL host, or Home)"},
 			{Name: "check", Description: "Validate the docs and write nothing", Bare: true},
 		},
 		Run: func(ctx context.Context, in bonfire.Input, out bonfire.Output) error {
@@ -91,6 +93,8 @@ func docsServeCommand() bonfire.Command {
 			{Name: "root", Description: "Repository root; src= paths and modules/ resolve against it", Default: "."},
 			{Name: "src", Description: "Docs source directory (default <root>/docs)"},
 			{Name: "base-url", Description: "Base URL for site links (overrides site.yaml base_url)"},
+			{Name: "site-url", Description: "Main site URL linked from the docs header (overrides site.yaml site_url)"},
+			{Name: "site-label", Description: "Label of the main site link (overrides site.yaml site_label; default: the URL host, or Home)"},
 			{Name: "addr", Description: "Listen address; must be loopback unless --allow-remote", Default: docsite.DefaultServeAddr},
 			{Name: "allow-remote", Description: "Allow a non-loopback --addr (serves the docs on the network)", Bare: true},
 		},
@@ -126,6 +130,8 @@ func docsOptions(in bonfire.Input) docsite.Options {
 	opts.Src, _ = in.Flag("src")
 	opts.Out, _ = in.Flag("out")
 	opts.BaseURL, _ = in.Flag("base-url")
+	opts.SiteURL, _ = in.Flag("site-url")
+	opts.SiteLabel, _ = in.Flag("site-label")
 	return opts
 }