docs: Adds docs folder

[grant]

docs: Adds docs folder

This PR adds all guides that used to live on https://developers.google.com/api-client-library/python/ to this repo. The content is not changed besides change the HTML into md. Note: Some of content may be out of date since most of it was authored in 2015.

Details

• Moves the API guides from the old site to this repo.
• Formats all docs in markdown (instead of HTML) for easy manipulation
• All docs are now open source, meaning anybody can comment and fix them!
• Adds a link to the homepage to the docs folder.
• Adds a docs/README.md navigation homepage.

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B: #669 #670 #671 #672 #673 #674 #675 #676 #677 #678 #679 #680 #681 #682 #683 #684 #685 #686 #687 #688 #689 #690 #693.
R: @busunkim96

[busunkim96]

Thank you @grant!