Revive missing illustrations in gh-pages branch

Fixes #102.

Images are taken from the docbook branch with the following mapping:

- chapter1.gif: chapter-about.gif
- chapter2.gif: chapter-tour.gif
- chapter3.gif: chapter-phonology.gif
- chapter4.gif: chapter-morphology.gif
- chapter5.gif: chapter-selbri.gif
- chapter6.gif: chapter-sumti.gif
- chapter7.gif: chapter-anaphoric-cmavo.gif
- chapter8.gif: chapter-relative-clauses.gif
- chapter9.gif: chapter-sumti-tcita.gif
- chapter10.gif: chapter-tenses.gif
- chapter11.gif: chapter-abstractions.gif
- chapter12.gif: chapter-lujvo.gif
- chapter13.gif: chapter-attitudinals.gif
- chapter14.gif: chapter-connectives.gif
- chapter15.gif: chapter-negation.gif
- chapter16.gif: chapter-quantifiers.gif
- chapter17.gif: chapter-letterals.gif
- chapter18.gif: chapter-mekso.gif
- chapter19.gif: chapter-structure.gif
- chapter20.gif: chapter-catalogue.gif
- chapter21.gif: chapter-grammars.gif