Managed repository

This PR tries to complete the FS Managed Repository page of the developer docs.

I have not copied across the Import Workflow section of the gdoc as this reads more like an overview of the import workflow from the client user rather than the developer. Instead I have taken some of that and written it as an introductory overview in lieu of a diagram, for which there is still a question. 

The server-side sequence has been copied in as-is though some of the conditional text in there might need removing. 

I think the sections could well be re-ordered, Configuration Properties doesn't make sense as a first section. Any comments on this would be welcome. Also, the Model Description section is marked as in-progress but this applies to the whole page at present so this warming could be either removed or moved to the top to replace the current in-transition text.

Finally, I have tidied up some of the bullet point nesting in the existing text.