DOC: Create a separate "book" for NUT QAA related chapters

[jimklimov]

"NUT Quality Assurance Automation" is not exactly user or developer documentation, so some chapters currently delivered in those may be better grouped in a separate collection:

• NUT CI farm (config-prereqs, ci-farm-lxc, finalization of CI: Use VMs provided by DigitalOcean FOSS support effort, and document the lessons learned #2192)
• ci_build.sh (partially targeted at developer workflows, partially at CI)
• NUT QA (details of make check, spellcheck, stylecheck, maybe valgrind)
• NIT (NUT Integration Testing suite)