hotdoc: Fix job definitions for documentation sites

Both the job templates in the hotdoc/ folder and the packaging/apertis-build-packaging.yaml template ended up generating jobs named {project}-{distro}/packages/build-apertis-designs and {project}-{distro}/packages/build-apertis-docs.

The hotdoc jobs weren't thus never enabled, with the packaging ones overwriting them. The hotdoc sites are still kept up-to-date by the old multi-branch jobs manually created a long time ago.

This commit fixes them up by:

• using a template rather than two mostly-identical job definitions
• naming the jobs {project}-{distro}/hotdoc/hotdoc-{package}
• updating the git URLs after the migration to GitLab
• adding the appropriate credentials to clone from GitLab

Test jobs available in my playground.