docs: Web Auth documentation (cadence-workflow#1235)

### Summary 
Create web auth document that includes:
- a brief on how backend auth works
https://github.com/cadence-workflow/cadence
- How auth is implemented in web
- How to use web auth

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Co-authored-by: Adhitya Mamallan <adi1998is@gmail.com>

Author: Assem-Uber