The 'resync' top-level rule now attempts to pull a set of READMEs and
guides from a maintainer's GIT repository which path has been passed
along with the --enable-doc-build[=<git-URL>] configuration switch.

Those files replace their counterpart in doc/asciidoc, before the
documentation is built then copied to doc/prebuilt.

NOTE: the original links are not preserved yet in the resulting
documentation.