enhancement(vrl): Add VRL function get_vector_timezone (#19727)

* Add VRL function get_vector_timezone()

This function returns the name of the configured timezone, or if set to
`local` then it will attempt to get the timezone name from the host OS.
If that fails, then it will get the local time, and return the formatted
timezone offset (e.g., "+02:00")

Note that iana-time-zone was previously an indirect dependency, now it's
a direct dependency.

* Add changelog fragment, fix cue format

* Ignore spelling of authors in changelog.d fragment files

* Doc updates from code review

* Add newline to changelog.d entry

* Refactor PR to just the documentation changes

* Changes from code review