Currently the background of the insert paragraph slug on desktop doesn't
have sufficient contrast with the text. Follow the pattern of what we
did in If03069339261e380dc49defedccb01830d162483, and update it to use a
higher transparency
Bug: T367822
Change-Id: Ia8f26da068f9b9f33f64523eee6c7f4bf64efaf7
Using PinnableElement in the TOC allows us to reuse styles associated with all PinnableElements, and helps ensure consistency between page tools and the TOC. This is something that would probably need to be done for T324505 anyway
Rather than updating PinnableElement to accept a class prop, I chose to rename '.sidebar-toc' to '.vector-toc', making the TOC's naming conventions consistent with the rest of Vector.
"sidebar" doesn't make sense anymore anyway, as the TOC can be in multiple locations and it's best to avoid the "sidebar" term
Bug: T324877
Change-Id: I5b9228380f5c4674ef424d33127a5cb4010822da
