Sensor model and reference system documentation (#36)

* Unfinished draft of sensor-models.md

* Addressed PR feedback and added text

* Removed incorrect DN description

* codespell and gramma

* Mermaid sequence diagram and requiremens setup

* Added figures and citations

* Updated glossary to include ISD

* Moved all quick definitions to top of sections, reworded some sections

* Address doc PR feedback

* Split ecosystem docs into separate file

* gramma and codespell

* Updated software link description

* Replaced todo with 'further docs' message

* Checkout glossary.md