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### Motivation
https://callstack.github.io/react-native-paper/activity-indicator.html
Gif not showing up in activity indicator example.
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### Test plan
Fixed the name of the gif file.
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While i was trying to update React and Flow versions, i found a small issue in docs. We were trying to access `android` and `ios` properties on wrong object.
* feat: initial implementation of banner component.
* feat: add props description.
* refactor: Change the way banner is structured.
* refactor: small refactor of banner component and example.
* refactor: remove some not needed logic from banner and add example.
* refactor: refactor of banner props description and little clean up in component.
* refactor: simplify implementation of banner
* feat: add snapshot tests for banner.
* feat: add banner gif.
* fix: fix example.
* fix: reduce banner gif size.
* fix: reduce gif size even more.
I've noticed that link to the _getting started_ page in the docs seems to not working as expected since `*.html` extension is added anyway and it makes the link broken:
`https://callstack.github.io/react-native-paper/getting-started.html.html`.
### Motivation
The documentation is important part of the library and should be working properly.
### Test plan
Enter `/docs`, install using `yarn` and run it with `yarn start`.
> **NOTE:** _the current version works on localhost and breaks only in the prod env. Suggested change will probably work in both, but needs to be verified by someone who knows the deployment process._
