Improve pipeline docs (w/ example code) - closes #134 (#255)
* Add example code for zero shot image classification
* Add example code for text classification pipeline
* Fix links to custom usage from pipelines docs
Reported on discord https://discord.com/channels/879548962464493619/1142943169068154950/1142943169068154950
* Fix relative links
* Rename .mdx -> .md
GitHub recently changed how mdx files are displayed, breaking a lot of the formatting. So, we just use .md now (same as transformers)
* Add example code for token classification pipeline
* Add example code for fill-mask pipeline
* Add text2text and summarization pipeline examples
* Add example code for image segmentation pipeline
* Remove redundant `@extends Pipeline`
* Add example code for image-to-text pipeline
* Cleanup example code outputs
* Cleanup JSDoc
* Cleanup pipeline example code
* Update codegen example