refactor(api-markdown-documenter): Bundle Markdown and Html renderers into namespace exports

[Josmithr]

Reduces clutter in the API surface by scoping render functions.

New usage looks like the following:

import { MarkdownRenderer } from "@fluid-tools/api-markdown-documenter";
...
MarkdownRenderer.renderApiModel(...);

Breaking Change

This PR removes a number of free functions from the global namespace. Consumers will need to access via ApiItemUtilities.

[alexvy86]

The changes look fine. I have a bit of an issue with the structure of the readme though. There's a level 4 heading for MarkdownRenderer under Usage->Document Generation, then a level 3 one under Architectural Overview, and no corresponding ones for HtmlRendering... but there's a level 2 heading for HtmlRenderer. Seems like we could restructure it a bit at some point.

[Josmithr]

The changes look fine. I have a bit of an issue with the structure of the readme though. There's a level 4 heading for MarkdownRenderer under Usage->Document Generation, then a level 3 one under Architectural Overview, and no corresponding ones for HtmlRendering... but there's a level 2 heading for HtmlRenderer. Seems like we could restructure it a bit at some point.

I've been trying to make improvements here when I find downtime. I'll make a point to go over the README next time I'm here.

Thanks for the review!