Add a full workflow in the docs

[ipopescu]

Corresponding documentation request: casper-network/docs#1299

New content:

• An installation Quickstart (quickstart-guide.md)
• A full installation guide with a brief review of the contract implementation (full-installation-tutorial.md)
• A new file on querying the NFT contract (querying-NFTs.md)
• A brief note on testing (testing-NFTs.md)

Modified content:

• Moved the content from “using-casper-client.md” file to “interacting-with-NFTs.md”, and added a section for minting and transfers using the entrypoints. Kept the wasm calls the same. Added a “checking the balance” section. Added “Approving an Account” section. NOTE: the wasm client calls keep failing for me, and I need to investigate, but I haven't modified the commands. All the new content I added was tested and working.
• I moved the “tutorials” folder under “docs”; this doesn’t seem to affect any of the docs.casper.network links.
• Updated old links

A PR is also open in the docs repo to update the content there.

[ACStone-MTS]

Looks great to me!

[gRoussac]

Approved. Thanks for your changes !

[ipopescu]

Thank you for the reviews, @gRoussac and @ACStoneCL!