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

[mpacer]

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

[mpacer]

Those tags are derived from their use in https://github.com/mpacer/hiding_tags_nbconvert.

[rgbkrk]

xref: jupyter/notebook#534

xref comment: jupyter/notebook#534 (comment)

[vidartf]

There is a line in the paragraph on cell metadata. Is this sufficient, or were you thinking something explaining it in more detail?

[rgbkrk]

I started to spec part of the tags in #134, though the discussion never coalesced so I've abandoned it. Anyone is free to pick it up!