Update the Upgrading landing page

## Overview

The "Upgrading etcd clusters and applications" page seems redundant. The "Upgrading" link could goes straight to the [main upgrade page](https://etcd.io/docs/v3.5/upgrades/), which contains the same information. There's no need to ever visit the [Upgrading etcd clusters and applications](https://etcd.io/docs/v3.5/upgrades/upgrading-etcd/) page.

It might be worthwhile to do a little research and remove or archive unneeded and unsupported upgrade pages. Also, is an upgrade path over more than one point release supported? If so, does it require a series of upgrades, such as for example 3.3 > 3.4 > 3.5? If so, state this explicitly on the "Upgrading" landing page. If not, document the supported paths.

Audience: Operator

Type: Task

The existing documentation might contain helpful source material that you can pull into this doc change. See recommendations for the existing (at the time of the CNCF tech doc analysis) etcd technical documentation pages:
https://github.com/cncf/techdocs/blob/main/assessments/0010-etcd/etcd-implementation.md/#general-reorganization

## 🎤 Context

This issue tracks recommended changes resulting from an analysis of the etcd documentation commissioned by CNCF. The analysis and supporting documents are here: https://github.com/cncf/techdocs/tree/main/assessments/0010-etcd/

## ✌️ Possible Implementation

Related material in the current doc:
- https://etcd.io/docs/v3.5/upgrades/upgrading-etcd/
- https://etcd.io/docs/v3.5/upgrades/

Suggested changes:

Remove the "Upgrading etcd clusters and applications" page and remove the link from the "Upgrading" main page. At the top of the "Upgrading" main page, document which upgrade paths are supported, recommended, deprecated, and unsupported. Provide instructions for upgrading over more than one point release, if necessary.