Finetune guide for semantic segmentation
This PR creates a finetune guide for semantic segmentation in the docs. Unlike previous finetune guides, this one will include:

* metrics for evaluation
* a section for how to use the model for inference
* an embedded Gradio demo

🚧 To do:
- [x] create section for inference
- [ ] create Gradio demo
_The documentation is not available anymore as the PR was closed or merged._