Moved community-supported integration to GH README

[cwarnermm]

As part of the consolidation effort to move all Mattermost-supported integrations to the Mattermost Product Documentation site, this PR moves the community-supported Mattermost Google Calendar integration documentation from Mattermost Product Documentation to this GH repository to a new /docs directory. Also updated the README with links to the new repo pages:

• About the Mattermost Google Calendar integration
• Set up the Mattermost Google Calendar integration
• Configure the Mattermost Google Calendar integration. Moved additional configuration settings over as well.
• Use the Mattermost Google Calendar integration