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`

width added to images for HTML rendering

See merge request OPAL/documentation/manual!15

(cherry picked from commit fd3db488)

66137798 width added to images for HTML rendering

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.