svg: gray time-info subtext not aligned (a fix: -Tsvg:cairo)

[tfiers]

After 4a5bcd7 (using html for gray subtext w/ "[$time ms]"),
noticed svg looks wonky (PNG is fine)

• https://gitlab.com/graphviz/graphviz/-/issues/1426 (wise words of steven c north)
• https://graphviz.org/doc/info/command.html#-T
• https://graphviz.org/docs/outputs/

btw, document that :pdf works nicely too :)
(and add link to the above outputs page)
Maybe a new arg (see #-T link above): renderer (:cairo, or nothing)

[tfiers]

-Tsvg

-Tsvg:cairo

(and -Tpng for good measure)

[tfiers]

the cairo svg draws each character as a path (it does dedup: has a library of 'glyphs' (letters & numbers) at the top it reuses).

• can you still have links? (urls: clickable nodes, i.e. svg: clickable nodes (urls to repos) #24)
• our darkmode posprocessing don't work here no more (the svg source is all different; no separate "node" groups etc anymore)

another possible fix: more postprocessing of the svg. like in the graphviz gitlab thread: "align center and set x to center of node"

[tfiers]

also lol click the above svg's to open as standalone images in firefox. both are effed up.
png good old just works.

if you want both light and dark, you can, with png, btw:
(documenter too?)
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to
i.e.

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="dark.png">
  <source media="(prefers-color-scheme: light)" srcset="light.png">
<picture>

(the extra (nonDRY) img attribute is for: 1) accesibility? can't picture or source have alt text? 2) backwards compat?. Is it necessary?)

[tfiers]

Hm maybe the edges should be gray in the above. (They're too noisy. Scaling (average of neighbour relsizes?) was first idea, but is hardee)