Docs: Add preprocessor

[sh-rp]

Description

We want to run all markdown pages through a preprocessor which injects snippets and other links. This will make snippets inserting easier and will remove code duplication for the most part. Re-organizing the snippets will come in a next step.

This PR does the following:

• Run all .md files in /docs through a preprocessor and saves them to /doc_processed from where they will be served, this folder is not committed to git.
• Removes the dependency on python for simply starting the docs. The api reference will not be available without explicitly building it. This should make it easier for contributors.

[netlify]

✅ Deploy Preview for dlt-hub-docs ready!

Name	Link
🔨 Latest commit	41fa986
🔍 Latest deploy log	https://app.netlify.com/sites/dlt-hub-docs/deploys/65fd42b83deba80008ed5b6a
😎 Deploy Preview	https://deploy-preview-1114--dlt-hub-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[AstrakhantsevaAA]

Sehr gut!
I love that we don't need to re-generate docs/examples locally before commit