Hacktoberfest page needs to be updated with evergreen language and links

[azigler]

https://handbook.mattermost.com/contributors/contributors/guidelines/hacktoberfest-mattermost-organizer-guide

This page currently just points to the Hacktoberfest landing page, but isn't a hyperlink. This page should be updated to have the following links, along with copy that can remain on this page for multiple years:

• Hacktoberfest landing page
• ~Hacktoberfest channel in Mattermost Community