Content sources as data processing pipelines

_Note: This issue is WIP and needs to be expanded._

## Notes

- Differentiate between **input data** and **output data**
- Input data validation could be done via Zod
- Question: How do define the output schema? (based on the input schema)
- Other schema formats: #163
- #250
- Also see [this comment](https://github.com/contentlayerdev/contentlayer/issues/214#issuecomment-1170856380)
- This would also unlock #84
- Make it easy to let the user define what they want to access (e.g. pre-rendered HTML vs raw MD source)

### Markdown/MDX processing pipeline

- This could allow for a smoother unified/remark/rehype content processing integration
- #137

## Related

- #120
- #163
- #106
- #84
- #160
- #91
- #275
- #86
- #71
- #148
- #24
- #149
- #26
- #216
- #225
- #325
- [Private Notion doc](https://www.notion.so/schickling/Build-pipeline-explorations-8214b31765bb48b9814d4dfaad23dfe8)