doc: Generate cross-references in code snippets to the Guile manual.

doc: Generate cross-references in code snippets to the Guile manual.

Occurrences of 'list', 'cons', etc. in code snippets now link to the
relevant section of the Guile manual.

* doc/build.scm (guile-manual, %guile-manual-base-url)
(guile-mono-node-indexes, guile-split-node-indexes): New variables.
(for-all-languages, merge-index-alists): New procedures.
<top level>: Rename 'mono-node-indexes' and 'split-node-indexes' to
'guix-mono-node-indexes' and 'guix-split-node-indexes' respectively.
Add 'mono-node-indexes' and 'split-node-indexes' defined as calls to
'merge-index-alists'.