Refresh ReadMe (usage example)

[tfiers]

• local image is now first class
  (so do that first, and give the depgraph_web and url as an alternative below)
• image of / section on loadtime :)

for darkmode, can use png's, as in

• svg: gray time-info subtext not aligned (a fix: -Tsvg:cairo) #103 (comment)

ok no, I'll do svg for first (clickable links :)), and png for second

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local image is now first class

it's maybe even only/main?
like, end-user API becomes just the one func, depgraph.
Why still use an online one, if dot is now bundled.
it's fun for live-playing w/ dot styling etc; but not so useful for end users I think
So: yes:

• depgraph_image → depgraph (only end user func). Relegate _web to internals/advanced/tools/.. #105

ah, hm, ok. Web is still useful for SVG zoom-and-pan (as written before here somewhere..)
otoh, we're veering more towards png again (which are also locally very zoom-and-pannable)