📝 Update / simplify the Getting Started vignette

[francisbarton]

Should close #137

This PR:

• Adds content on how to work with Azure table storage
• Differentiates content on Azure authentication from that on set up of environment variables
• Removes the {keyring} examples as superfluous (users can use this package if they like but it's not really relevant to how azkit is designed to work)
• Adds links to some external resources
• Simplifies some of the included code to remove unnecessary examples.

See issue text for rationale for these changes.

The PR also:

• adds an .Renviron.example file to the repo
• fixes an error in list_files()