doc: Add liblink specification

doc/src/04100-lib-link.md

@@ -0,0 +1,42 @@
+# liblink {#sec:liblink}
+
+The "liblink" library contains functionality for linking from entries to
+internal and external content.
+
+The utilities provided by "liblink" contain functions to
+
+* Get internal links from arbitraty entries
+* Access an "EntryHeader" object to
+  * get internal links
+  * set internal links
+  * get "Entry" objects for links from a header
+
+## Linking internal content with liblink {#sec:liblink:internal}
+
+As one entry MAY contain zero or more interal links in the header section
+"imag.links", the "liblink" library provides functionality to
+
+* get the link from an EntryHeader object
+* set the links in a EntryHeader object
+* query for a specific link in a EntryHeader object
+  * by regex
+  * by module name
+  * by filename
+
+as well as functionality to get "Entry" objects for each entry in the Header.
+
+## Linking external content with liblink {#sec:liblink:external}
+
+As one "EntryHeader" MUST NOT contain more than one link to external content (as
+defined in @sec:thestore:linking:external, the following scheme for linking to
+external content MUST BE provided by "liblink":
+
+* Generate a "link" entry in the store
+  * with store path starting with "/link/"
+  * where the header field "imag.content.uri" MUST BE set
+  * with optional content which can be stored in the section of the "liblink"
+    module section (section name "link", as defined by
+    @sec:thestore:fileformat:header:module).
+* Get an external link by store path (looking up the store path entry and
+  getting the link to the external content from it)
+