gen-apidocs: ByKind keyed by name only causes cross-group IsOldVersion false positives

[Caesarsage]

Problem

Definitions.ByKind in generators/api/definition.go is keyed by kind name only:

for _, d:= range s.All {
    s.ByKind[d.Name] = append(s.ByKind[d.Name], d)
}

When two unrelated API groups define the same kind name, they share a bucket. The next loop sorts the bucket and flags everything past index 0 as IsOldVersion, which is wrong when the entries belong to different groups.

Example HTML backend

Take TypedLocalObjectReference as a concrete case. It exists as two unrelated kinds:

• core/v1.TypedLocalObjectReference
• scheduling.k8s.io/v1alpha2.TypedLocalObjectReference

They share a name but are independent types. With name-only ByKind keying, the
generator treats them as versions of each other.

Master (buggy):

The scheduling.k8s.io/v1alpha2 page shows an "Other API versions of this object
exist: v1" alert linking to core/v1.TypedLocalObjectReference: a misleading
claim, since the two are independent kinds.

With the fix:

The misleading alert is gone. Each kind is treated as its own bucket.

Proposed fix

Key ByKind by (group, kind) instead of kind alone:

type byKindKey struct {
    Group string
    Kind  string
}

type Definitions struct {
    ByKind map[byKindKey]SortDefinitionsByVersion
    ...
}

FindNewestVersion(group, kind) already takes a group; manual filtering becomes unnecessary. Same v1.36 spec, after the fix: zero cross-group false positives.

PR follows.

[Caesarsage]

/assign