Missing table of contents on pages

[allisonport-db]

For example on https://docs.delta.io/latest/delta-batch.html we no longer have the table of contents. It's extremely hard to navigate the longer pages and find what you want without this.

[nkarpov]

Leaving here for later.

There's a table of contents plugin from Gatsby but it has a bug where it weirdly renders:
signalwerk/gatsby-remark-table-of-contents#33

Options:

1. Make a custom component in this repo
2. Fix that bug in gatsby-remark-table-of-contents

For (2), can bundle it here inline to not have to deal with an external PR.