Add meta tag to allow inter-documentation links

For now the only way to refer another part of the doc (that doesn't belong to the current book) is to use the "static" and full URL.
For example :

We can think to add a meta tag (eg. %DOC_ROOT% and maybe also %DOC_SECTION_DEVGUIDES%, %DOC_SECTION_GETTING_STARTED%, ...) to make this reference more dynamic.
The above example will become :