Link the 19 API-key access-review connector docs

Each of these providers ships an API-key connection path and a driver, but
no Registration carried a DocumentationURL, so the console rendered no
"how do I get this token" link on the connect dialog. probo.com now has a
page per provider, so point each Registration at it.

Slugs are passed explicitly rather than derived from the enum, per
accessReviewDocsURL's contract, so BETTER_STACK maps to better-stack and
INCIDENT_IO to incident-io.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/provider/mercury.go

@@ -31,9 +31,10 @@ import (
 
 func mercuryRegistration() *Registration {
 	return &Registration{
-		Provider:       coredata.ConnectorProviderMercury,
-		DisplayName:    "Mercury",
-		SupportsAPIKey: true,
+		Provider:         coredata.ConnectorProviderMercury,
+		DisplayName:      "Mercury",
+		DocumentationURL: accessReviewDocsURL("mercury"),
+		SupportsAPIKey:   true,
 		// Mercury authenticates with a self-serve API token presented as
 		// Authorization: Bearer, the default APIKeyConnection scheme. There
 		// is no third-party OAuth2 flow for the Users API. The token is