[Feature Request] Symbol when an article has a different primary TOC

**Is your feature request related to a problem? Please describe.**

When the same article is listed in two or more `TOC.yml` files (tables of contents), DocFX chooses one of those as the primary TOC that will be displayed when a user views that article. This can result in a jarring change when the user is browsing articles in one TOC and ends up seeing another TOC without having expected that to happen.

**Describe the solution you'd like**

In the table of contents, show 🔗 or another symbol next to the name of the TOC item, to warn the user that the current TOC is not primary for that article and selecting the article will cause a different TOC to be displayed.

**Describe alternatives you've considered**

I can edit the `TOC.yml` files and append 🔗 symbols to the `name` properties of TOC items, but:

* It is easy to forget to do this to a TOC item. An automatic build-time solution would be more reliable.
* That causes the symbol to be underlined as part of the hyperlink on hover, and I'd rather keep the symbol outside the hyperlink. (If it were injected using HTML `class` and CSS `::after`, that would make it easy to customize.)
* The symbol also appears in the "Next" and "Previous" links that the "modern" template generates below neighbouring articles. I'm not sure whether that is good or bad.
* There might be a risk of those symbols ending up in the breadcrumb bar too, although perhaps that can't happen.

**Additional context**

The choice of primary TOC can currently be controlled via the `order` property, which is documented in .

Start by tracing how TOC.yml entries are processed and rendered in the modern template, paying attention to primary-TOC selection and the generated Next and Previous links. The change is complete when non-primary TOC entries display a customizable symbol outside the article hyperlink without incorrectly affecting breadcrumbs or other navigation.

Written by the indexing model from the issue text.