Bug: ToC should be scrollable

[homersimpsons]

Description

Long "Table of Content" are not realy usable as they require the user to scroll to the bottom of the page in order to be able to use the bottom entries.

Instead of this the "Table of Content" should have a fixed height and a scroll overwflow so it is scrollable independently from the page.

Steps to Reproduce

1. Open https://ossf.github.io/osv-schema/ (on a "wide enough" screen)
2. Notice that the "Table of Content" on the right is not really usable.

Expected behavior

The "Table of Content" should be scrollable