Add API documentation

[robin-nitrokey]

To do:

• Review module structure #9 so that we have less module pages with more content
• rename all module pages to only the module name
• remove unnecessary subsections
• remove package/module distinction
• consider moving api/nitrokey.rst into api/index.rst
• add more doc strings
• add usage guide
• add docs checks to CI

[robin-nitrokey]

Marking as ready for review – we still have virtually no doc strings, but I think that should not block the documentation infrastructure.

[robin-nitrokey]

See this PR to build and preview the SDK docs as part of nitrokey-documentation: Nitrokey/nitrokey-documentation#269