contributor docs: Extend help center writing style documentation.

[alya]

Current: https://zulip.readthedocs.io/en/latest/documentation/helpcenter.html#user-interface

Updated:

[alya]

Trying to run a linter just hangs for me (I tried tools/lint --only prettier --fix docs/documentation/helpcenter.md to minimize what's being run), so I dunno how to fix this PR.

[timabbott]

Fixed the linter error and merged. For reference, the Prettier issue was:

diff --git a/docs/documentation/helpcenter.md b/docs/documentation/helpcenter.md
index c573f3acc1..ed42389122 100644
--- a/docs/documentation/helpcenter.md
+++ b/docs/documentation/helpcenter.md
@@ -220,7 +220,7 @@ is needed. In particular:
 - Do not list out the options the user is choosing from. Once the user finds the
   right menu in the UI, they'll be able to see the options. In some cases, we
   may describe the options in more detail outside of the instructions block.
-- Never identify or refer to a button by its color. You *can* describe its
+- Never identify or refer to a button by its color. You _can_ describe its
   location.
 - Use screenshots only when it's very difficult to get your point across without
   them.