[docs] Add documentation for login command

Feature to login with command `evalai login` was implemented here: https://github.com/Cloud-CV/evalai-cli/pull/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/