docs: add Tencent Cloud webhook provider

[QianChenglong]

What this PR does

• Adds external-dns-tencentcloud-webhook to the webhook provider registry table in README.md
• Creates a dedicated tutorial at `docs/tutorials/tencentcloud.md`

Why

The in-tree Tencent Cloud provider was removed per the webhook migration plan. This webhook implementation supports both DNSPod (public DNS) and PrivateDNS (VPC-scoped) with two credential modes:

• Static: SecretId/SecretKey — works on any Kubernetes cluster
• OIDC: TKE RRSA — keyless authentication for TKE managed clusters

References

• Webhook repo: https://github.com/tkestack/external-dns-tencentcloud-webhook
• Provider registry section in `docs/tutorials/webhook-provider.md` encourages PRs to add provider links

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[mloiseleur]

@QianChenglong Usually, most of the webhook providers keep their tutorials on their repository. That said, I do not have a strong opinion on that, and I'm ok with that. So let's see what the other maintainers think about this.

/ok-to-test

[coveralls]

Coverage Report for CI Build 25711968114

Coverage decreased (-0.005%) to 80.583%

Details

• Coverage decreased (-0.005%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 1 coverage regression across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
openshift_route.go	1	82.93%

---

Coverage Stats

Relevant Lines:	21378
Covered Lines:	17227
Line Coverage:	80.58%
Coverage Strength:	1452.69 hits per line

---

💛 - Coveralls

[mloiseleur]

/lgtm

[ivankatliarchuk]

Why we need a webhook tutorial docs/tutorials/tencentcloud.md in this repo? You could have up-to-date tutorial in the webhook repo, and keep it up to date against different external-dns versions