[dicts] clean up DTDs, add/improve documentation

[Phaqui]

1: Make sure that the .dtd file only exist in giella-core, and that each dictionary refers to that one. An immediate observation
is that the path to the dtd in .xml files are written as a relative path. After the move to github, users may have the dictionaries cloned to an arbitrary path. This could make running xmllint --valid on a dictionary challenging.

2: Document what the different attributes mean, how they are used, and what they are used for. There are two audiences for such documentation:

• Dictionary authors. They need to know how to express various linguistic properties in the dictionary. For example how to mark transitive/intransitive verbs, noun cases, etc.
• Programmers: who will need to know how the various pieces are going to be used, or presented. For example, how to display the fact that a verb can only be used in a transitive way.

[Phaqui]

Comments on 1:

• The file is /giellalt/giella-core/dicts/scripts/gt_dictionary.dtd, and dictionary files reference it using a relative path.
• However, some dictionary has their own dtd, in a dtd/xxxyyy.dtd file.
• One possible fix to the path problem: Put the dtd on a publicly accessible url. This does however mean that an internet connection is required to validate against it.
• Another fix is to require all users to keep repositories belonging to the same organization in the same folder.