Improved documentation

[mdklatt]

• [x] Upgrade Sphinx configuration.
• [x] Document new API in README.
• [x] Improve security information in README.
• [x] Add a PlantUML data flow diagram to README.
• [x] Add developer notes to README

[mdklatt]

Issue resolved by commit b1557edb3b1567627e1638a1e1e75c820ec99bf9.