🐛 webhook: clarify CertDir default and wrap serving-cert load error

[alliasgher]

Summary

Addresses the two small, agreed-upon asks from #900:

1. Better docs. The `CertDir` default — `/k8s-webhook-server/serving-certs` — depends on `os.TempDir()`, which is `/tmp` on most Linux installs but is `TMPDIR`-dependent elsewhere. Maintainer asked to keep the default stable but update the comment to explain this and to nudge operators toward setting CertDir explicitly (usually to a mounted Secret path). Done.
2. Helpful error. When `certwatcher.New` fails because the certs weren't where `CertDir` was pointed, users got a bare `open ...: no such file or directory`. Wrap the error so it names the paths that were attempted, prints the effective `CertDir`/`CertName`/`KeyName`, and points at `webhook.Server.Options.CertDir` as the thing to fix. The underlying error is preserved via `%w`.

Before

```
open /tmp/k8s-webhook-server/serving-certs/tls.crt: no such file or directory
```

After

```
failed to load serving cert from "/tmp/k8s-webhook-server/serving-certs/tls.crt" / "/tmp/k8s-webhook-server/serving-certs/tls.key" — did you mount the certificate files at webhook.Server.Options.CertDir? (CertDir="/tmp/k8s-webhook-server/serving-certs", CertName="tls.crt", KeyName="tls.key"): open /tmp/k8s-webhook-server/serving-certs/tls.crt: no such file or directory
```

Fixes #900

Tests

`go build ./pkg/webhook/...` and `go vet ./pkg/webhook/...` pass. No behavior change except the added context on the error path.

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[k8s-ci-robot]

Hi @alliasgher. Thanks for your PR.

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[troy0820]

/ok-to-test
/hold

[alliasgher]

/retest

[k8s-ci-robot]

@alliasgher: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-controller-runtime-test	e31aa60	link	true	/test pull-controller-runtime-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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