Possibility to override sourceLink parameter

We need to amend the links to the page sources, in order for them to be valid for Apertis Pro specific pages. With this feature, we get the possibility to do so.

Features:

• .Site.Params.sourceLink defines the hostname for the page sources
• sourceLink is hard-coded to the Apertis Gitlab repository
• for downstream Apertis pages, the hard-coded sourceLink is invalid
• this commit allows to override sourceLink using a local page parameter
• such parameters will be added to pages specific to downstream
• "View page source" renamed to "Edit on Gitlab"
• state that page source can be changed via GitLab, not just viewed

Signed-off-by: Sietze van Buuren Sietze.vanBuuren@de.bosch.com