Edit URLs

The core issue is that we need to configure edit_url manually. Normally, the edit_url is made up by the repo url and branch. However, our url is just a strange file path, and the branch HEAD. These values are not usable.

Got things working for oesenc_pi in current master. The source def in site.yml is

- url: ./sources/oesenc_pi
  branches: HEAD
  start_path: manual
  edit_url: https://github.com/leamas/oesenc_pi/edit/master/{path}

Here, the paths are also somewhat simplified. Nevertheless, the edit_url stuff forces us to write a long and scary path.

NOTE!!! things does not work without defining the environment variable CI when running antora. The value does not matter, export CI=true is the standard thing.

I suggest that we start adding edit_url attributes only to plugins which are properly maintained i. e. we are not using a private fork for the manual.