I have been trying to find a way to have a table of contents of the subpages within the Azure DevOps Wiki. I have found, from other wiki services, how to do it.
In Confluence, they have a macro for "Children Display"
Another way I found for another wiki service: [Special:PrefixIndex/Help:Subpages/]
I am afraid there is not exactly the same feature(Confluence: Children Display) in Azure Devops.
In Azure Devops, the wiki page has the Table of contents(TOC) markdown.
This feature is similar to Children Display, but it can only connect to its own child pages.
It has limitations and cannot connect to subpages other than its own page.
If you want to connect to other subpages, you need to use link markdown.
But it cannot add a table of contents of the subpages.
Therefore, the features in the Azure Devops wiki cannot meet your requirements for the time being.
Here are the suggestion tickets in our UserVoice Site.
Wiki macro for displaying child pages
Table of contents should consider subpages
Many Users have the similar requirements, you could vote and add comment in the suggestion tickets.
You can also work around the issue by putting links in # headers after the [[_TOC_]].
##### [Name of page](/path/to/page)
Then it will show up in your table of contents. I also use this at the very top of a sub page to navigate back to home. If you put this before the [[_TOC_]] it won't be in the table but will still show up at the top.
##### [Back to home page](/Home-Page)