Document support for The Things Gateway Controller

[johanstokking]

Summary

Documentation for https://github.com/TheThingsIndustries/lorawan-stack/pull/4464 and https://github.com/TheThingsIndustries/lorawan-stack-aws/pull/1199.

Changes

• Document ttgc section for self-hosted
  • Since one can use ACME certs for TTGC, also revised the TLS sections
• Document managed gateways for AWS AMI
  • Also updated other parameters and defaults

Notes for Reviewers

This should not be merged before we merged the referenced PR and know which version this will land.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.