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/sendgrid.go

@@ -31,10 +31,11 @@ import (
 
 func sendgridRegistration() *Registration {
 	return &Registration{
-		Provider:       coredata.ConnectorProviderSendGrid,
-		DisplayName:    "SendGrid",
-		ProbeURL:       "https://api.sendgrid.com/v3/teammates?limit=1&offset=0",
-		SupportsAPIKey: true,
+		Provider:         coredata.ConnectorProviderSendGrid,
+		DisplayName:      "SendGrid",
+		DocumentationURL: accessReviewDocsURL("sendgrid"),
+		ProbeURL:         "https://api.sendgrid.com/v3/teammates?limit=1&offset=0",
+		SupportsAPIKey:   true,
 		NewDriver: func(_ context.Context, c *http.Client, _ *coredata.Connector, logger *log.Logger) (drivers.Driver, error) {
 			return drivers.NewSendGridDriver(c, logger.Named("sendgrid")), nil
 		},