internal/experiment: create command-toc experiment

Creates an experiment to enable the table of contents
for command documention pages.

For golang/go#38073

Change-Id: I192a1b523b6df0c96dc16602a6fc8898c7ad2069
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/288552
Trust: Jamal Carvalho <[email protected]>
Run-TryBot: Jamal Carvalho <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: jamalc

internal/experiment.go

@@ -6,6 +6,7 @@
 package internal
 
 const (
+	ExperimentCommandTOC           = "command-toc"
 	ExperimentDirectoryTree        = "directory-tree"
 	ExperimentNotAtLatest          = "not-at-latest"
 	ExperimentRedirectedFromBanner = "redirected-from-banner"
@@ -14,6 +15,7 @@ const (
 // Experiments represents all of the active experiments in the codebase and
 // a description of each experiment.
 var Experiments = map[string]string{
+	ExperimentCommandTOC:           "Enable the table of contents for command documention pages.",
 	ExperimentDirectoryTree:        "Enable the directory tree layout on the unit page.",
 	ExperimentNotAtLatest:          "Enable the display of a 'not at latest' badge.",
 	ExperimentRedirectedFromBanner: "Display banner with path that request was redirected from.",