Rename "service discovery" / "single controller deployments" to "gateway discovery"

[mflendrich]

Is there an existing issue for this?

• I have searched the existing issues

Problem Statement

Kong/charts#747 (comment) proposed a "gateway discovery" name for the #702 feature.

Proposed Solution

Conclude the aforementioned discussion from the Helm chart PR and:

• either rename this feature to "gateway discovery" as proposed, or
• drop the proposal

Additional information

No response

Acceptance Criteria

• documentation refers to the SCD feature as gateway discovery
• CLI flags refer to the SCD feature as gateway discovery
• Helm chart refers to the SCD feature as gateway discovery

[czeslavo]

As there's no documentation on docs.konghq.com yet, all ACs seem to be satisfied. I've created an issue to track docs separately: #3604