added permanent-ulr page

docs/webapp/permanent-url/index.rst

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+Permanent URLs
+================================
+
+InterMine generates stable and unique URLs to identify the report pages for biological entities.
+They are based on class names combined with local IDs provided by the data resource providers and therefore they are persistent. 
+In FlyMine, for example, the URL of the report page for the gene zen, with primary identifier P19107, will be http://flymine.org/gene:FBgn0004053.
+They are used to share the report page with other users.
+
+
+The class_keys.properties file speficies the keys used to generate the permanent URLs.
+If not specified, the primaryidentifier key is used.
+
+=============== ================
+key             value
+=============== ================
+Protein_URI     primaryAccession
+Organism_URI    taxonId
+Publication_URI pubMedId
+=============== ================
+
+
+.. index:: permanent url