Restructure Install Elastic Agents in a containerized environment section

[**Install Elastic Agents in a containerized environment**](https://www.elastic.co/guide/en/fleet/master/install-elastic-agents-in-containers.html) section needs to be modified:

- Change titles for each page to be more clear and to the point
- Remove everything that can be considered an extra
- Create a new tutorial for each extra
- Restructure cloud providers pages to _Important Notes_ and _Limitations_ (new)

Suggestions for improvement for each of these points can be found in [this doc](https://docs.google.com/document/d/1Cvu-4CPN5FhSMtCqo08UOVcjGh1UnF_JGolVjmNUARs/edit?usp=sharing). These include:

- Four new tutorials:
1. Vanilla Kubernetes
2. How to update integrations
3. How to deploy kube state metrics
4. Monitor Kubernetes with Elastic Agent (similar to [this page](https://www.elastic.co/guide/en/observability/current/monitor-aws-elastic-agent.html)
- Change titles like **Run Elastic Agent on GKE standard managed by Fleet** to **GKE considerations**, for example
- Make warnings shorter

Start with the linked Install Elastic Agents in a containerized environment section and the Google Docs recommendations, then inventory its pages and cloud-provider content. Done means clearer page titles, shorter warnings, separate tutorials for the four listed topics, and cloud-provider pages reorganized into Important Notes and Limitations.

Written by the indexing model from the issue text.