move diagrams out of doc directory

[dlech]

Since not all generated diagrams are used for just the docs anymore, it would be nice to move docs/main/diagrams_source to a top-level directory (I can never remember where to find it). Also, any generated diagrams not used by the docs could be placed in the appropriate place, like npm/images.

[laurensvalk]

When we do this, we could consider exporting the optimized SVG instead of PNG.

And when we do that, maybe it’s worth having a fresh look to see if Sphinx can handle those too.