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`

Using code blocks to render OPAL output seems to be better than literal
blocks. The same is true for excerpts of the OPAL input file.

Due to a bug in the asciidoc2pdf converter script the last line of a file
must be terminated with a newline. To ensure this a 'EOF' comment has been
added to all files.