Improve `MeshRetry` docs and add missing `MeshHTTPRoute` targetRef

[bartsmykla]

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Description

The retryOn section in the MeshRetry docs is hard to understand. It’s not clear what each option means or how they work together. For example, when I tried to check if 503 responses were covered, I had to look at the Envoy docs because our docs didn’t help. The explanation about combining status codes (5xx, 429) and methods (HttpMethodGet) was confusing and made it seem like they couldn’t be used together.

Also, add MeshHTTPRoute should be added to the supported top level targetRefs

Suggested Fix

Update the retryOn section in the MeshRetry docs to clearly explain what each option does and how they can be combined.

Example Policy

type: MeshRetry
mesh: default
name: retry-policy
spec:
  targetRef:
    kind: Mesh
  to:
  - targetRef:
      kind: MeshService
      name: service-name
    default:
      http:
        retryOn:
        - Reset
        - ConnectFailure
        - HttpMethodGet