Use doc builder v4

[BrennanGit]

This includes a few changes to support the new doc_builder:

• doxygen multiple includes replaced by single inclusion of doxygengroup and :c:func: to create references
• change top-level of programming guide from includes to a toc
  • required change in doc builder to remove README from default exclusions list
• change name of settings file in lib checks (conveniently, version remains on line 4, but it's not guaranteed)
• Jenkinsfile update to make parallel docs build

https://xmosjira.atlassian.net/browse/AP-150

[shuchitak]

Looks great!

[BrennanGit]

Bare in mind, v4 hasn't been released yet so we'll need to update the image tag once it's done.
Also as per discussion in lib_xcore_math, I'm not sure it's worth zipping up the docs as we lose the ability to render them.