internal: add experiment for retractions and deprecations

For golang/go#43265
For golang/go#41321

Change-Id: I1e8e6fe0f55d536696e1c58e81726292278fd20c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/295891
Trust: Jonathan Amsterdam <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>

Author: jba

internal/experiment.go

@@ -12,6 +12,7 @@ const (
 	ExperimentInteractivePlayground = "interactive-playground"
 	ExperimentNotAtLatest           = "not-at-latest"
 	ExperimentRedirectedFromBanner  = "redirected-from-banner"
+	ExperimentRetractions           = "retractions"
 )
 
 // Experiments represents all of the active experiments in the codebase and
@@ -23,6 +24,7 @@ var Experiments = map[string]string{
 	ExperimentInteractivePlayground: "Enable interactive example playground on the unit page.",
 	ExperimentNotAtLatest:           "Enable the display of a 'not at latest' badge.",
 	ExperimentRedirectedFromBanner:  "Display banner with path that request was redirected from.",
+	ExperimentRetractions:           "Retrieve and display retraction and deprecation information.",
 }
 
 // Experiment holds data associated with an experimental feature for frontend