[IMG303] Add gui development guid to dev_docs

Zhang, Chenrequested to merge
IMG303_devdoc_ui into next
  • Original Gitlab Enabler: IMG303

This PR adds a developer doc on GUI development guide in dev_docs. To build and view the page, go to dev_docs root and issue make html. If sphinx is properly installed locally, the html documentation should be ready in dev_docs/build.

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