Clarify the Linux Getting Started guide (#9346)
Clarify the Linux Getting Started guide (#9346)

- Prerequisites: Make the requirement for a DNS server a bit
  more generalized.
- Add an admonition box directing users expecting a local
  evaluation setup to our Docker Compose guide.
- Add a bit more context around how ACME works
- Move "Configure DNS" before "Configure Teleport" to provide a
  clearer order of tasks.
- Add a bit of explanation for why a user would create DNS
  records.
- Add some explanation for how "teleport configure" sets up
  TLS.
- Add a "Start Teleport" H3 section to separate this step from
  previous ones.
- Various minor additions to add clarity.

Closes #9077 since it refers users without access to a DNS server
to the Docker Compose guide.

Closes #9083
gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport