Clarification of procedure for using docs in local Grav required

[dcsjapan]

I noticed some problems with the organization of the first page (Gantry Admin) in the Configuration section, and thought I could help out by contributing some changes. I naturally wanted to verify my changes before submitting a PR, since GitHub cannot render the graphic elements of the docs properly.

So, based on the info in the Readme, I forked the develop branch, cloned it locally, and set up a local Grav installation to use the local repo as the user folder. When I tried to open the local Grav site in my browser, I got the text for the first page of the docs preceded by this error message:

ERROR: chapter.html.twig template not found for page: /

Clearly something is not working as it should. I'm not sure whether this is a problem with the Readme documentation for using the docs locally, or if there is something missing in the docs repo. In any case, the Readme instructions for setting up the docs locally are inadequate, as they consist of the single sentence:

If you would like a local copy of the documentation, you can either download it or you can clone the repository by running the following command:

More detailed instructions for setting up the docs in a local Grav installation would be very helpful. Making this process easier to navigate will greatly increase the chances that Gantry users will want to contribute to the project.