Document tags, including possibly jupyter native tags, but definitely mentioning namespaces

Tag documentation for nbformat is pretty lacking. We're talking about explicitly supporting `jupyter:hide_input`, `jupyter:hide_output`, `jupyter:hide_cell`, `jupyter:kill_input`, `jupyter:kill_output` and `jupyter:kill_cell`, though that's not necessary.

Downside to namespaces is that they make the tags much longer which pollutes tag-space.

@ellisonbg @rgbkrk @minrk

No file or test is identified in the issue. Start by reviewing the existing nbformat tag documentation and the proposed jupyter:* names, then resolve the namespace scope and document the agreed tag behavior.

Written by the indexing model from the issue text.