Is Section editing possible in ikiwiki? Is there a plugin?
Not exactly the same, but a large document is sometimes stiched together by inlining smaller sections. The sections can then be clicked on to go to their page and edited. For example, see http://git-annex.branchable.com/walkthrough/. A side benefit of this is that users can post comments on each section of the page, which are visible when viewing the page for that section, but not at the top level.
I suspect section editing is necessary on eg, Wikipedia because it reduces the change for conflicts when multiple people are editing different parts of the page. Ikiwiki instead avoids conflicts by harnessing the merging power of a version control system such as git, so multiple edits to different sections of a page can be made and each saved without manual conflict resolution.
I'm happy that modern web browsers allow searching inside text edit boxes, so when I want to jump to a given part of a large page I'm editing, I just search for the text.