Change to H2 for TOC

We currently use H1 in our document pages to build the TOC on the right hand column. To follow semantic best practices we should only have one H1 on the page (the title) and use headings starting with H2 for structuring page content.

We would need to configure Wyam to pickup H2 for building the TOC:

```
Pipelines["RenderPages"].InsertAfter("WriteMetadata", new Headings(3); // Heading level
```

Search the Wyam configuration for the RenderPages pipeline and the WriteMetadata insertion point shown in the issue. Confirm how the Headings level controls the right-column TOC, then verify that rendered document pages use H2 for content structure while retaining a single H1 title.

Written by the indexing model from the issue text.