Landing pages for all left header sections

[gingerwizard]

Every landing page should have one, if not auto generate one based on its content.

cc @kellytoole P1

[Blargian]

@gingerwizard PR 2779 and 71846 add landing pages for left headers where they were missing to make breadcrumbs clickable. I will make some updates to those PRs so that the landing pages have autogenerated TOC where necessary.

[gingerwizard]

@Blargian yes ideally we generate a page if an index.md doesnt exist, based on the listing of items. These wont have descriptions so a list design maybe better for now.