Draft: Reenable docs & coverage

• Docs reenabled
  Chose option() name ENABLE_DOC after checking what other UBports/Lomiri projects are using. <PROJECT_NAME>_ENABLE_DOC_GENERATION is used more commonly, but I think this could get very unwieldy with some of the longer project names.
  • Doc installation uses CMAKE_INSTALL_DOCDIR as discussed in !19 (merged)
• Coverage reporting reenabled
  Enabled by using CMAKE_BUILD_TYPE=Coverage (casing doesn't matter). The usual make -> make test -> make coverage should work again.