Move widoco to another file, create an Ontology description section (#48

)

* Rewrite Abstract, introduction and title

* Use <#thing> instead of :thing in the examples

* Fix terminology index keyword

* Replace targeting by referring to

* Insert Widoco section and change 3.1 index description

* Add intro text at the top of the vocabulary section

* fix invalid turtle in example 4

* add missing prefixes (they are used in examples)

+ the RDF namespace is the one for the core vocabulary,
not for RDFS

* better (IMO) definition of index

(loosely based on https://en.wikipedia.org/wiki/Database_index)

* improve ontology

- better labels
- prefix are hermit-friendly
- hasTarget is no longer functional
- add axioms to make hasTarget and hasSubIndex exclusive
- add axiom for hasCount to require hasSubIndex

* Allow only one value for hasTarget and hasSubIndex
We need only one value to use hasCount with hasSubIndex.

* Move the vocabulary section to an external file

* Update widoco from ontology