[web] Fixed search index generation by using only PNGs.

Jenkinsfile

web/content/books/BMB-3-Cover.png

web/content/books/bmb-3.pandoc

web/content/books/bmb-4.pandoc

web/content/books/bmb-4.png

web/content/books/computational-geotechnics-i.pandoc

web/content/books/computational-geotechnics-i.png

web/content/books/geoenergy-modeling-ii.pandoc

web/content/books/geoenergy-modeling-ii.png

web/content/books/geoenergy-modeling-iii.pandoc

web/content/books/geoenergy-modeling-iii.png

web/content/books/models-of-thermochemical-heat-storage.jpg

web/content/books/models-of-thermochemical-heat-storage.png