Update documentations (#12247)

* Update README.md

-Added link to license
-Added link to code of conduct

* Update install_mac.md

Replace OS X with macOS

* Update index.md

Replace OS X with macOS

* Update install_mac.md

Remove bad practice of sudo pip install <package>

* Update README.md

Give example to beginners who should close sessions when not in use

* Update recurrent.md

Made explicit HTTPS calls

* Update deep_cnn.md

Made explicit HTTPS calls

* Update image_recognition.md

Made explicit HTTPS calls

* Update image_retraining.md

Made explicit HTTPS calls

* Update kernel_methods.md

Change HTTP link to equivalent link in HTTPS

* Update layers.md

Made explicit HTTPS call

* Update seq2seq.md

Made explicit HTTPS calls

* Update using_gpu.md

Made explicit HTTPS call

* Update wide.md

Made explicit HTTPS calls

* Update wide_and_deep.md

Made explicit HTTPS calls

* Update word2vec.md

Made explicit HTTPS calls