v2306: New and improved usability

The previous Extended Code Guide has been migrated from a Doxygen-based system to nanoc-based system (the same system as used by Gitlab.com) and made public for Community Contributions. The new content is available from https://doc.openfoam.com:

• v2306: https://doc.openfoam.com/2306/

• v2212: https://doc.openfoam.com/2212/ (old release)

The site has a new look and feel, and has received significant content updates, including a quick start, many new boundary conditions, thermophysical models, tools including noise, and much more!

The content is hosted at https://gitlab.com/openfoam/documentation - it is continuously evolving and further updates will follow.

We welcome your feedback, and encourage you to get involved - we're happy to receive pull requests!

Expression evaluation now includes normal() as a function. This returns the unit normal of mesh faces - a more robust equivalent to face()/area() with fewer operations.