doc update how to reset a wiki project from a Rails console

Problem to solve

• What product or feature(s) affected?

Project wiki's

• What docs or doc section affected? Include links or paths.

GitLab Rails Console Cheat Sheet

Further details

It can sometimes be required to reinitialise (create a new empty wiki) for a Project.

Proposal

Add Rails console command to do this.

Who can address the issue

Support

Other links/references

ZD: https://gitlab.zendesk.com/agent/tickets/138900 (internal use only)