Fix heading hierarchy so pages normalize off h2 as the base heading level

[cjcenizal]

Every page in Docusaurus has a page title that is automatically an h1. Many of the pages in our docs define headings using h1 as the base heading level. This results in numerous h1 elements on the page, which is an accessibility violation and also prevents the page heading navigation from populating:

See #72 for an example of how to fix this issue.