Include centralized alternative sourceLink option

This is an add-on to this merge request

I realized that using the feature added in the original merge request was cumbersome:
On every page with a different sourceLink I had to include the full alternative sourceLink. This is error-prone (copy-pasting the same link into many files is going to cause problems sooner or later) and also is not very elegant.

Therefore, this merge request will introduce the ability to add a centralized alternative sourceLink, defined in a single location, which can be enabled, simply setting a parameter named useAltSourceLink to true in the page's front matter.

• option to override sourceLink w/ altSourceLink defined in config.toml
• enabled if page parameter useAltSourceLink is defined and true
• only used if altSourceLink is defined
• defining sourceLink in front matter overrides all other (alternative) sourceLinks

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