[Feature Request] Pin TOC filter at the top on scroll to page bottom

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

When browsing the docfx site, and scroll to the bottom of the page.
Then `footer` area is displayed at bottom.

At this time, the TOC/Affix area is scrolled. and it hide `Textbox for TOC filter`.

**Describe the solution you'd like**

The TOC/Affix navigation area is not scrolled whether the footer area is displayed or not.

**Additional context**
Current behavior is the same as the [Bootstrap 5.3 official documentation site](https://getbootstrap.com/docs/5.3/getting-started/introduction/)

This behavior is not caused when using `default` template.