Based on @ellisonbg's research in gh-2914, most tools with similar
functionality to run a piece of code use either the 'play' icon, or
something custom designed (RStudio and Spyder are in the latter camp).
The analogy with audio and video does suggest that 'step forward' is
more similar to running one cell, but it's not clear that users
are actually thinking about it in those terms.
I'm opening this for discussion rather than for immediate merge.
This makes the high-res screenshots larger on a lower-res screen, but
nbsphinx doesn't currently handle HTML <img> tags, so we need to use the
Markdown format to show images in our docs.
Closes gh-2304
Thomas Kluyver
M Pacer
Carol Willing
joonnifer
Fernando Perez
Matthias Bussonnier
Sylvain Corlay
Jonathan Frederic