Document kas support for observability TLS

What does this MR do?

Mention in the docs that TLS can be configured for kas. There is no special config exposed as this is not a very common thing to want to do, but I think it's worth mentioning that it is actually supported rather than saying it is not.

Related issues

It was added in %15.4 in gitlab-org/cluster-integration/gitlab-agent!728 (merged) for gitlab-org/cluster-integration/gitlab-agent#288 (closed).

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.