[docs] Add documentation for login command

[nikochiko]

Feature to login with command evalai login was implemented here: #138
But the documentation lacks a reference to the login command.

As for its location in the documentation, I think having it in the "Get Started" section would be most appropriate. Also, I think it should replace the set_token and get_token commands there. (Any feedback here?)

Suggesting changes:

• Add documentation for login command and appropriate instructions (in Get Started section)
• Move the documentation for set_token and get_token to another section (maybe a new section Configure Token)

Link to documentation: https://evalai-cli.cloudcv.org/

[devil-cyber]

Can i work on this issue?

[RishabhJain2018]

Hi @devil-cyber, Sure!

[devil-cyber]

Thanks

…

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