Initial migration of in-place import docs

This is a copy of the ome-internal documentation
(InPlace.pdf and ImportScenarios.pdf) to the main
sphinx codebase. The changes required for the
migration were:

 * exclude `downloads` because of the *.txt ending
 * change `:omerodoc:` to `:doc:`
 * prepend a few slashes for absolute URLs

 Otherwise, the content has not (yet) been modified.
Better import scenarios introduction
Further title modifications
Add links to in-place-import
Rework in-place intro
Add proper reference links
Extend limitations section
Remove 'docs are not yet public' section
Remove repetitive 'have' prefix
Fix bullet indent
Add ref link to dropbox section
Link to in-place from scenarios
Add future development section
Plans, accounts & wording tidying
Add import files to what's new list
Fix missing '-' from file name!
Merge pull request #724 from joshmoore/rebased/develop/in-place-docs

In-place documentation (rebased onto develop)