grim/hgkeeper

Remove the explicit write access of admins to the hgkeeper repo.

Fixes HGKEEPER-21
This documentation gets deployed to
[docs.imfreedom.org/hgkeeper](https://docs.imfreedom.org/hgkeeper/) via CI.
To build it locally it requires the
[include-markdown](https://github.com/mondeja/mkdocs-include-markdown-plugin)
plugin.