Implement script/makefile to create new AGL doc website

New AGL docs website is now split into several repositories :

https://github.com/automotive-grade-linux/docs-webtemplate
https://github.com/automotive-grade-linux/docs-sources
https://github.com/automotive-grade-linux/docs-tools

A script or a makefile must allow to easily setup and generate AGL doc website.
Needs:

• script / makefile must be part of docs-webtemplate repo  (root repo to clone to generate AGL docs)

• support multi branches (based on a config file that differs from a branch to another) in order to allow generation of master or master-next docs

• clone docs-tools repo in the right location

• install requested dependencies when need

  • check if node_modules dir exists under docs-tools, if not run "npm install"

  • check if jekyll and other requested tools are install, if not rise an error with the right command line to execute (eg. sudo zypper install ruby ... )

• fetch docs-sources documentation part as any other external docs repo (IOW add docs-sources in yaml fetch file)