Now each page is indexed by their title (by default), and each index
term declared for it on the index
2 manuals are included, the data import/export as its own category (as
it is rather short), and each top-level section of the R introduction
manual (as it is quite a bit longer).
Add some manual cleanup.
Some pages still seem missing:
- either belonging to non-default packages, i.e. it is normal that they miss
- or corresponding to index words without their own package (!)