Table of Contents section numbers

This is minor, but the Table of Contents has section numbers that don't always match what's below. For example, the Table of Contents has a link that says:

> [3.2.3 Subsurface](https://academysoftwarefoundation.github.io/OpenPBR/#model/basesubstrate/subsurface)

Although the number in the ToC here says 3.2.3, the section it leads to is numbered 3.7.3.

In fact it appears that there are no sections numbered 3.1 to 3.5. Section [3 Model](https://academysoftwarefoundation.github.io/OpenPBR/#model) is followed immediately by section [3.6 Microfacet Model](https://academysoftwarefoundation.github.io/OpenPBR/#model/microfacetmodel) which is labeled as 3.1 in the ToC.

Start from the OpenPBR document linked in the issue and compare its Table of Contents entries with the linked section headings, especially Subsurface and Microfacet Model. Correct the inconsistent section numbers in the document source, then verify that the ToC labels and linked headings match.

Written by the indexing model from the issue text.