New documentation based on GitBook CLI

Split the root README.md in multiple Markdown files (in the `docs/` folder) to make easier reading, editing and extending the documentation. An online version is also available on netlify (https://qtpromise.netlify.com). Building it requires Node.js installed, then:

- npm install -g gitbook-cli
- gitbook install ./
- gitbook build . dist/docs

.gitignore

@@ -1,3 +1,5 @@
+dist
+node_modules
 *.gcno
 *.gcda
 *.moc
@@ -9,4 +11,5 @@ Makefile*
 moc_*.cpp
 moc_*.h
 coverage.info
+package-lock.json
 target_wrapper.bat