See also the official documentation:
Help Center > Owners Manual > Pinning topics and wiki pages to the Topics page

Using a "Sticky wiki page" to post announcements in your Messages section

Using a Sticky Wiki page can be used to, as the title suggests, post announcements in your Messages section.

Another use is to promote your wiki by making it the first thing they see on the Messages page.

NOTE: You must be an Owner or Moderator to make a Sticky Wiki Page

To make a Sticky Wiki page:

1. On the left hand sidebar, click on Admin
2. From the drop-down menu, click on Settings
3. Scroll all the way down to the bottom of the Settings page
4. The next to the last setting is Sticky Wiki Page
   • By default it reads (no page)
5. Click on the arrow at the right-hand end of the Selection Drop-down
6. Select the wiki page you want displayed at the top of your Messages page
7. Click the Update Group button to save your changes

NOTE: Only the top 5 lines of your wiki page will be displayed, so make them count.

At the bottom of your Sticky Wiki Page banner there is a "Read more" link that will take them to the full wiki page.