docs: added link to assertion count section in assertion page

[DevMoustafa97]

Summary

It was my first time to read the documntation and i found something called assertion.count() so instead of serching for it i found it will be great if it was a link to click on it to navigate directly to its documentation so i decided to do it by myself added a link to assertion.count() where it is mentioned to make it easy for the reader to get more info about it

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• :white_check_mark: login: DevMoustafa97 / name: Moustafa Mahmoud (2522a2089ecd8fc75c6874b11037d642b8483770, 105465ba6f8715d771be72b9f3f895263346cbea)

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