update docs to encourage earthaccess.login() without strategy

[JessicaS11]

v0.5.0 added functionality for the login function to automatically try all of the strategies without requiring the user to specify each. The docs need to be updated to reflect this simplification for the user.

[jrbourbeau]

Thanks @JessicaS11. #300 updates earthaccess to handle auth automatically if there's a ~/.netrc file around or EARTHDATA_* environment variables set. We also updated the authentication section in the docs to encourage earthaccess.login() (w/o strategy=) when ~/.netrc or EARTHDATA_* env vars around specified.

I'm going to close this issue as fixed, but let me know if I'm missing something and there are other places you think should be updated too.