Adds a shared file with general information about Zulip's
translation project based on the text in web-app's language
picker, and uses that text at the beginning of the help articles
for setting the organization notifications/announcement language
and for the user setting their personal language setting.
Also makes some small edits/updates to the help center article
about the user's personal language setting to align with current
UI and current documentation styles.
Alya Abbott
Lauryn Menard