Use a general mechanism of storing all job variables for each build job
separately, so that we have all documentation files in the
FILES_documentation variable. This makes the dedicated HTML_FILES
variable obsolete.
The merged variables are still kept, though, so that we have all
machines in the MACHINE variable to loop over.

Previously the deployed files were stored in a dedicated text file
called files.txt. Move this information to a dotenv variable called
FILES, so that we don't need special handling for it in the Confluence
stage.

This adds two jobs to the Yocto pipeline:

 - generate-confluence-page
 - publish-confluence-page

The first one generates a confluence page from a template
(`confluence-page.xml.jinja2`) using information downloaded from all
successful "Deploy FTP" jobs of the same pipeline.

The second one publishes this page in Confluence and displays a link to
the new page. If the page already exists it is overwritten.

The place in Confluence where the new page is published is configurable
via the `CONFLUENCE_SPACE` and `CONFLUENCE_PARENT_ID` variables.