Add GitHub actions to generate / clean CJSON and SVG

[ghutchis]

Now that #36 is merged, it would be useful to have some GitHub actions that run on modifications to:

• generate any missing files
• clean and flatten CJSON files
• clean and flatten SVG (i.e., svgcleaner)
• etc.

This would make it easier for someone to add a bunch of SMILES fragments and have GitHub generate the files and depictions, etc.

[nbehrnd]

With revised depict_ligands.py in hand, a difference view shows some functional overlap with rdkit-fragments. If memory serves well, the generation of .png by the later no longer is needed for Avogadro2 (if true, then depict-polymers.py and eventually README.md of the fragments repository equally are affected). Eventually, file thiocyanate.png might become the last bitmap this repository retains.