Add netrc environment variable into the documentation (#1675)

Add netrc environment variable into the documentation (#1675)

* Adding netrc environment variable documentation

* Update docs/environment_variables.md

Co-authored-by: Tom Christie <tom@tomchristie.com>
* Modifications about netrcfile environment variable

* Change uppercase "A" in netrcfile env variable into lowercase

* Added some words and a dot before "my_netrc" in the console example

* changed a typo "rather that" into "rather than" in advanced.md

* changed netrc environment variable in example part

* modified title for netrc environment variable part in doc

* Deleted the dot in title of netrc environment variable

Co-authored-by: Tom Christie <tom@tomchristie.com>