Removed docs folder and deleted base-content tag (#269)

## Issue
Discourse gatekeeper fails due to the page before "Reference" (i.e.
("Minor rollback") being too far nested

## Solution
As a temporary workaround, added a new page "[How to
contribute](https://discourse.charmhub.io/t/how-to-contribute/14528)" in
between "Minor rollback" and "Reference":

Workflow still fails because it's using the previously generated index.
This PR is to reset the docs by deleting the `/docs` folder and removing
the `discourse-gatekeeper/base-content` tag.

Author: a-velasco