Generate the PNG from SVG programmatically

[dpordomingo]

Caused by #447 (comment)

It is needed to decide how to create PNG versions of SVG to be used in docs (because GitBook does not support SVG).

Currently it is done with a screenshot from mermaid view, but it does not scale and can not be integrated in the CI.

[dpordomingo]

alternatives from #447 (comment)

ImageMagick

convert \
    docs/assets/lookout-sdk-seq-diagram.svg
    lookout-sdk-seq-diagram.png

@carlosms said at #447 (comment)

(this way) they look worse, the text typography or kerning makes it harder to read.

There are some other alternatives, that render better PNGs (specially CairoSVG), but we should find the better options to do it.

svgexport

$ svgexport docs/assets/lookout-sdk-seq-diagram.svg \
    docs/assets/lookout-sdk-seq-diagram.png \
    "svg{background:white;}"

Inkscape

$ inkscape -z -e docs/assets/lookout-sdk-seq-diagram.png \
    --export-background white \
    docs/assets/lookout-sdk-seq-diagram.svg

CairoSVG

$ cairosvg -f png --output-width 900 \
    --output docs/assets/lookout-sdk-seq-diagram.png \
    docs/assets/lookout-sdk-seq-diagram.svg