* Add a guide to deploying an HA cluster
Closes#16751
This is a general guide that prefaces our HA deployment guides.
Also adds an introduction to the "deploy-a-cluster" section.
* Respond to some PR feedback
- Add mentions of specific services (Auth and Proxy) where it would help
with clarity.
- Edit port tables to remove ports that shouldn't be publicly exposed.
- Clarify the supported backends.
- Clarify that Let's Encrypt is not required for TLS credential
management.
* Respond to more PR feedback
- Add context around DNS records
- Use "Layer 4" instead of "Layer Four"
- Explain optional ports more explicitly
- Fix spacing issues
- Indicate that you can use an S3-compatible object store
- Clarify cert-fetching behavior for applications
- Describe separate port configs depending on whether TLS Routing is
enabled, and add a brief section re: whether to enable TLS Routing
- Expand the Teleport configuration section to accommodate TLS Routing
and separate listeners
* Add more context to the "Deploy a Cluster" intro
* Small language tweaks
* Add an image
* Respond to zmb3 feedback
- Use "cluster state backend" and "session recording backend"
- Mention cross-zone load balancing
- Link to the Backends Reference instead of including example backend
configurations
- Use v3 for example configs
- Fix example config indentation
* Add CTA for Teleport Cloud and forScopes
