Fixes#35131.
Most of the nginx config is copied from
https://docs.astro.build/en/recipes/docker/#nginx.
We ideally should be generating two builds for Zulip cloud and serve a
different one depending on whether the subdomain is same as the root
domain or not. We can look into this as an immediate followup while this
commit helps things get ready for a test deploy on CZO.
@types/mdast was in dev dependencies while it shouldn't have been, we
move that away from dev dependencies and bump the version. We need the
package when running the astro build step.
We are starting the cutover process and starlight_help is the directory
we have agreed on to place our new help center project. We do not want
to use `starlight_help` as the URL for the project, but this commit
changes the url from `help-beta` to `starlight_help` temporarily since
we can only change URL once we get rid of the current help center
project. That will be done in a future commit.
