Run automatic build of the Handbook folder

[uribench]

Call 'handbook build' command automatically to always keep the Handbook folder on the GitHub repository current with the contents (guides and topics) and config files.

Suggested by @richardapeters to eliminate conflicts between different contributions in which one contributor is building the handbook with contents of another contributor that did not build the handbook.

This automation can be done at various points along the pipeline from the 'git commit' to the local clone until the pull request on GitHub.