Add API reference for components (IEC-57)

[igrr]

Is your feature request related to a problem?

It is difficult to discover/learn the API of components which aren't ports of well-known libraries.

Describe the solution you'd like.

Component Registry now supports displaying the contents of API.md file, if it is provided in the component directory. We also have a pre-commit hook to generate API.md from Doxygen-generated XML files.

Describe alternatives you've considered.

• Find the header file of the component, read Doxygen comments
• Read examples (which are missing from most components...)

Additional context.

Example repo where API.md is set up: https://github.com/igrr/pa1010d

[igrr]

At the moment, doxybook only supports a single Doxyfile as input and a single API.md as output. We'll have to change this to make it usable for multiple components in the same repo.