Docs: Add guide for implementing custom markdown renderer

[brandonroberts]

Which scope/s are relevant/related to the feature request?

Docs

Information

The @analogjs/content package has a ContentRenderer abstract class that can be implemented and provided to use a different renderer for markdown content, such as remark with rehype

This needs to be documented to show how to create a custom renderer for those who want to use something other than the built-in one with marked and Prism.

Describe any alternatives/workarounds you're currently using

No response

I would be willing to submit a PR to fix this issue

• [ ] Yes
• [ ] No

[joshuamorony]

I'll be implementing remark/rehype shortly so I can create a PR for the docs once I'm done with that