Webmaster doc

[xendk]

Add documentation about configuration handling in webmaster modules.

[xendk]

I don't know why I get those merge commits, I've just rebased on origin/main.

[kasperg]

I don't know why I get those merge commits, I've just rebased on origin/main

Try rebasing against develop 😄. These are merge requests from develop into main. The PR is against develop.

[kasperg]

👍 Nice. I think this is well written.

There are some parts that I think needs to be addressed primarily usage of DPL and the document structure.

Please also consider if some parts actually belong in an ADR about configuration handling for custom modules for webmaster sites. This allows us to mention alternatives considered, known challenges, future perspectives etc. while this documentation only dealing with the practicalities and known processes.