Initial work to add FS import to developer docs

See ticket https://trac.openmicroscopy.org.uk/ome/ticket/10931 - this adds content from https://docs.google.com/document/d/15zpLHsXhvSbCa5hLIn9Mwj3spCq-2wvN9QXLZCao3ZU/edit# to the sphinx docs ready for updating & tidying. Some sections are still be added to FSlite.txt but this PR is intended to review current content and structure before the rest is filled in in later PRs.