MA live doc rendering: Support work items created by the user inside sections

[ewuerger]

The user wants to manage work items inside generated sections. These new work items need to be cached and recognized in the ordering. Headings managed from the template need a special id such that they can be distinguished from user created headings.

There are some special cases that need to be part of the test coverage:

1. If the work item above (anchor) is deleted, the previous work item above becomes the new anchor
   a. If more than one work items above are deleted the nearest neighbor (from above) is taken as the new anchor, even if it is in a different chapter (level). However the work item doesn't leave its chapter in this case.
   b. If the whole chapter is deleted and the section left empty from rendered work items then the work items managed by the user are moved to the "Orphaned work items section" below the autogenerated section.
   i. The orphaned work items section is introduced by an explaining paragraph in bold and red.