[docs] Use correct header heirarchy in airbyte-protocol docs

[erohmensing]

What

• Makes Actor Specification and other currently-h1 headers linkable, makes links to them resolve, and discoverable in the sidebar. See my issues with how things weren't working currently in this Loom

How

• Use h2 for current h1 headers and adjust accordingly.
• Context: Docusaurus only shows h2 and h3 headings by default, and though they're not explicit about it, don't encourage using h1 elsewhere in the page, since the title of the page is the h1 header.

We could also enforce/test this hierarchy if we wanted to, happy to create an issue if people think that would be helpful. Example here both checks that there are no other h1s in the doc and that the hierarchy doesn't skip a level.

[erohmensing]

Loom opened it before I was ready 😄