Promote IAC docs for agents and dynamic resources

[ptgott]

Closes #27382
Closes #25418
Closes #25442

Two aspects of setting up a Teleport cluster are omnipresent in the docs but have not received dedicated treatment:

• Running agents
• Applying dynamic resources

As a result, it is difficult to include discussions about those topics that are separate from a specific workflow or how-to guide. One glaring absence has been prominent guidance on using infrastructure-as-code tools to achieve these tasks.

This change improves the visibility of Teleport's support for infrastructure-as-code tools by:

• Creating docs sections for running agents and applying dynamic resources
• Making IAC instructions prominent within these sections

The intention is for readers to become familiar with different methods of applying dynamic resources and running agents, including how to do this with IAC, so they can apply this knowledge when reading other parts of the docs.

The new section on dynamic resources also gives us a place to put other guides to using the Terraform provider and Kubernetes Operator, e.g., if we add guides to using these tools with popular GitOps platforms.

Likewise, the section on agents gives us a place to put other agent-wide information, e.g., how to enable an additional Teleport service on an instance that is already running.

Substantial changes in response to feedback:

Move the "Dynamic Configuration" section

Make this a subsection of "Manage your Cluster" since it's not
self-evidently clear as a top-level docs section. Users will probably need an introduction via the "Manage your Cluster" section intro page.

This also reverts some of the more drastic sidebar changes introduced by the previous commit.

Responds to xinding33 feedback.

Make IAC learning tracks prominent/hard to avoid

Closes #27751

Responds to xinding33 feedback

The motivation is to be more opinionated about the course that users
take through the docs. We currently recommend two tracks for
self-service users, i.e., the users expected to make use of the landing
page:

• Setting up a toy self-hosted Teleport cluster
• Setting up a Teleport Team/Enterprise Cloud cluster that can
  eventually become production ready

By moving the "Get Started" guide to the landing page, we direct users
immediately on to the first track. This change then gives new users a
way to enter the second track from the docs landing page with a
prominent link to the Teleport Team docs.

This change also edits The landing page to direct users who have
completed the getting started experience to instructions for setting up
a pool of agents on Terraform, helping to make infrastructure-as-code a
first-class citizen of the docs.

This change also removes the menu of links that used to confront users
on the landing page. Since all sidebar sections include introduction
pages, users interested in the content of a sidebar section can visit
the section. By removing links, we make it clearer for users how to
proceed through the docs.

[ptgott]

TODO:

• Make sure I have added redirects

[alexfornuto]

👍 , though my local render fails on deploy-agents-terraform with a Expected a closing tag for ` error that I couldn't track down the source of.