doc: Add rake linkchecker task, and document it in the README file
* README: remove numbers in headings

* linkchecker: only check file:// links

* epydoc: run against the local source tree, not against whatever
  version of the python 'arvados' package is installed

* generate: re-run epydoc on every generation

* generate: do not fail to include the epydoc output files if they are
  generated in the current run (which they now always are)
Merge branch '2509-rake-linkchecker'