Model summary doc page horizontal banners
Author: mishig25
Pull request: yes

# What does this PR do?
Horizontally align model summary page banners

To achieve this results, there are 2 options:
1. Write html directly (as done in this PR)
OR
2. Create a specific svelte component & use it (something like `import SomeComponent.svelte` & `<SomeComponent banners={...}>`)

I think option2 is an overkill for this use case since option1 is a very simple html. @sgugger wdyt

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Fixes # (issue)
As mentioned in https://github.com/huggingface/doc-builder/issues/73#issuecomment-1006423852

compare "Original GPT" banners vs "GPT-2" banners

<img width="785" alt="Screenshot 2022-01-06 at 17 46 00" src="https://user-images.githubusercontent.com/11827707/148421166-f613b7d9-b4ed-4ef7-925b-35b9f647bba5.png">



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Comments:
cc: @AK391