[Docs] Align BanyanDB cluster/discovery docs, drop etcd leftovers, and publish docs from a docs branch

## Background

Follow-up from https://github.com/apache/skywalking/discussions/14077#discussioncomment-18431254: community hit contradictory port examples between the cluster install guide and the file-based discovery guide, and asked which gRPC port liaison should use.

Default `--grpc-port` for both `banyand data` and `banyand liaison` is **17912**. `18912` is only a common override when data and liaison share the same host/IP. Etcd-based discovery has been removed; docs still mention it in places.

## Tasks

### 1. Fix cluster / file-discovery port examples

- Align [installation/cluster](https://skywalking.apache.org/docs/skywalking-banyandb/latest/installation/cluster/) and [operation/node-discovery](https://skywalking.apache.org/docs/skywalking-banyandb/latest/operation/node-discovery/) so `nodes.yaml` / liaison examples are consistent.
- State clearly: default gRPC port is `17912` for data and liaison; when co-located on one IP, use different ports (e.g. data `17912`, liaison `18912`) and list the same ports in `nodes.yaml`.

### 2. Remove redundant etcd references from documentation

- Purge leftover etcd discovery / configuration / upgrade wording from BanyanDB docs (e.g. `docs/operation/node-discovery.md`, `docs/operation/upgrade.md`, and other hits under `docs/`).
- Keep only current discovery modes: `none`, `dns`, `file`.

### 3. Docs branch for continuous doc updates without a release

- Create and maintain a dedicated docs branch in https://github.com/apache/skywalking-banyandb (e.g. `docs` / `asf-site` workflow as appropriate) so documentation can be updated and published without waiting for an official BanyanDB release.
- Document how doc PRs should target that branch and how they reach skywalking.apache.org.

## Acceptance

- [ ] Port guidance is consistent across cluster + node-discovery docs
- [ ] No obsolete etcd discovery instructions remain in the published docs
- [ ] Docs branch exists and is the agreed path for mid-release doc fixes

Start with docs/operation/node-discovery.md and docs/operation/upgrade.md, then search the other files under docs/ for the remaining etcd references. Compare the published installation/cluster and operation/node-discovery pages to align the port examples. Review the repository's existing publication workflow before proposing the docs branch path; done means consistent port guidance, no obsolete etcd instructions, and an agreed publishable branch workflow.

Written by the indexing model from the issue text.