Update versioning and availability

[lcawl]

Relates to #2795

This PR updates the introductory Versioning and availability page.
It is a work in progress until we address the following feedback:

• Add new Elastic intro section #2795 (comment): "I don't know if this [Examples of where availability can vary] section is very clear. I think it could use some more details or introductory content. Somewhere on this page, we should try to explain a few things, how these multiple, differently versioned, products work together and how this can impact availability:
  • Elastic Cloud, ECE, and ECK are orchestrators used to create and manage serverless projects (Elastic Cloud only) and deployments of the Elastic Stack
  • The versions of the Elastic Stack that you can deploy depend on the version of these orchestrators. For example, Elastic Stack 9.0 is not available to deploy on ECE 3.1.
  • Because some products are released on different schedules and have different architecture or network constraints, it is possible that a feature isn't available for some deployment types, or becomes available at a different time for each deployment type."
• Add new Elastic intro section #2795 (comment) "we also do some of this explanation on the following surfaces and you can mine them for language / make sure they align:
  • https://www.elastic.co/docs/deploy-manage/deploy#about-orchestration
  • https://www.elastic.co/docs/deploy-manage/deploy#versioning-and-compatibility
  • https://www.elastic.co/docs/deploy-manage/upgrade/plan-upgrade"
• Add new Elastic intro section #2795 (comment) "> Because some products are released on different schedules... would be careful with this because a lot of functionality is controlled by the stack version. might indicate that these are usually at the admin layer rather than the storage/querying layer of elastic. examples would be helpful (e.g. not all auth types are available for all deployment types [e.g. ldap] - that's kind of a layer "above" / "around" the es engine)"

[github-actions]

🔍 Preview links for changed docs

• get-started/howto-use-the-docs.md
• get-started/versioning-availability.md
• versions.md

[georgewallace]

I'm good with the changes, I just have one question. If the content at the bottom is removed we need to find a proper place for it to not have loss of information.