[IMG123] Add developer documentations

Zhang, Chenrequested to merge
IMG123_init_devdocs into next
  • Original Gitlab Task: IMG123

This PR introduces the following changes:

  • update git ignore list for repo management
  • add new folder dev-docs to store developer documentation
  • add overview page for developer documentation.

The developer documentation is build with sphinx, and the html version can be built by running make html inside dev_docs.

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