remove hard-coded folder Name for RDF prefix file

[FTeichmann]

Trying to work with rdf Prefixes, I get many

template: rdf_index.html file not found:

errors as the prefix file is expected to be in a folder named 'rdf-data':

$ ack rdf-data
rdf_page_data.rb
61:          prefixFile=File.new(File.join(base, 'rdf-data', self.data["rdf_prefix_path"].strip)).readlines
68:          Jekyll.logger.error("context: #{resource}  template: #{template}  file not found: #{File.join(base, 'rdf-data', self.data["rdf_prefix_path"])}")

Please remove the expected folder name for better configurability.

[white-gecko]

@Simaris please verify and fix the documentation (README.md) about the convention about where the prefix mapping is expected.

[Simaris]

I did verify that the documentation is in this case clear (see $), but I don't think we should remove rdf-data, since it goes with Jekylls overall design pattern.

If it is really necessary to keep files outside Jekyll, the use of sym-links should do the job.