Add technical reference documentation

[ethanjli]

This PR adds various technical reference pages to our docs site, describing various aspects of the PlanktoScope software:

• Release process: describes our versioning system and release channels, focusing on information which would help users to understand how our release process may impact them. This page does not try to provide information specific to developers/maintainers of the PlanktoScope software (e.g. about how we set the scope of each release), since that information is probably not very helpful for users.
• Camera settings: describes how the Node-RED dashboard's exposed camera settings are used to control the camera. This includes an explanation of white balance settings, which closes docs/technical-reference: Explain how manual white balance gains are interpreted #357.
• Sample imaging: describes what "stop-flow imaging" is, and why we use it.
• Image segmentation: describes (to the best of my understanding from studying the code) what the segmenter does (which closes docs/technical-reference: Explain how the segmenter chooses which objects to keep and discard #359), and all the metadata fields it attaches to each segmented object (which closes docs/technical-reference: Explain all metadata fields exported by the segmenter #360).