fix: allow customers to define the slug in the metadata

[mjcuva]

🧰 Changes

Coinbase was running into an issue where pages were getting new slugs instead of updating the existing ones. They added a slug field in their metadata in the markdown files, which it seems like we didn't officially support. Since we were using the file name to generate the slug when seeing if a doc existed or not, but still created the page with the slug they defined it was ending up in a weird state where pages were being created on every sync.

🧬 QA & Testing

If you want to test locally you can run the following:

./rdme/bin/rdme docs ~/path/to/markdown --key KEY --version 1.0

This will update the page here: https://dash.readme.com/project/PROJECT/v1.0/docs/new-page

---
title: "New Page"
slug: "new-page"
category: "CATEGORY ID"
---
# This is a page with a slug defined

Look at that slug. 🐌 (not a slug, but the best I can do)