- tabs replaced with spaces in Manual.attributes
- unused definition of attribute 'baseurl' removed
- use attribute 'link_manual_src' instead of 'link_OPAL_doc'

All links to `gitlab.psi.ch` and `amas.web.psi.ch` are now defined as attributes in `Manual.attributes`. In a new branch, only this file must be changed to get the right links.

The header for rendering the chapters in `env-gitlab` is now defined in `env-gitlab.attributes`