Merge branch 'feature/docs_general_notes' into 'master'

Docs: new documentation and warnings cleanup

This change set

- adds a high-level description of application startup flow. Some parts are missing, but hopefully we can use this description as a base to expand on.
- adds a few notes about memory regions and their use in ESP-IDF.
- add SPI flash and partition APIs page
- fixes all Doxygen warnings in header files
- enables build failures on new Doxygen warnings

See merge request !201