Update Z api docs

[jxjj]

• add docs for updating zlink endpoint
• switch the javascript example
• sticky contents and header
• remove details on how JWT works. This is covered well in linked https://jwt.io page. Users should be using a library for signing their payload anyway.
• link to an example implementation

For review here: https://cla-z-dev-02.oit.umn.edu/shortener/api_keys

[cmcfadden]

looks good