Update doc img links
Author: mishig25
Pull request: yes

# What does this PR do?
Companion PR for https://github.com/huggingface/doc-builder/pull/34

TLDR: all doc img links (both rst & md) should be in format `/imgs/xyz.png`
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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
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