Hyperlinks together importc like pragmas.
Starts backends.txt with bits from nimrodc.txt.
Hyperlinks back JavaScript target.
Starts nimrod calling backend section.
Adds nimrod to backend examples.
Adds examples of backend calling nimrod.
Mentions memory management.
Adds tidbit about threads.
Rewords C derivate backend. Refs #1299.
Mentions nodejs in js intro. Refs #1299.
Merge pull request #1299 from gradha/pr_documents_interfacing

Documents backend interfacing