GH.io API pages build broken by #440

[evaleev]

I think #440 broke the API reference on the github pages build, the page now links to https://valeevgroup.github.io/.doxygen/html/index.html which gives me a 404, the old link https://valeevgroup.github.io/SeQuant/genindex.html is still there but is an empty table.

Originally posted by @obackhouse in #440 (comment)