More links in the HTML manual

[Ludovic Courtès]

Hi Guix!

I pushed the following changes to the post-processing code for the
on-line HTML copies of the manual:

--8<---------------cut here---------------start------------->8---
f54149062e doc: cookbook: Balance parens in custom kernel examples.
97ce30cc1b doc: Generate cross-references in code snippets to the Guile manual.
db1d445357 doc: Allow code snippets in the cookbook to link to the manual.
0f7d0743ed doc: Move manual index creation to a separate derivation.
--8<---------------cut here---------------end--------------->8---

This improves links for identifiers that appear in code snippets in two
ways:

  1. The cookbook now has links in its code snippets too (previously
     only the manual had that those).

  2. Both the manual and cookbook link to the Guile manual.  For
     instance, you can click on ‘cons’, ‘string-append’, etc. and that
     will lead you to their reference in the manual.

Examples:

  https://guix.gnu.org/cookbook/de/html_node/Ein-Hallo_002dWelt_002dPaket.html
  https://guix.gnu.org/manual/devel/en/html_node/Build-Utilities.html
  
https://guix.gnu.org/manual/devel/fr/html_node/Utiliser-le-systeme-de-configuration.html

Note that the link remain honor the current language and mono/split-node
mode.

Since this matches by string, there are a few false positives: ‘version’
links to the ‘version’ procedure (even though it’s the name of the
<package> field), ‘method’ links to a section about GOOPS, ‘bind’ links
to the ‘bind’ procedure (in an example where it refers to the package),
the identifier ‘system’ in (use-modules (gnu system)) links to the
‘system’ procedure, and so on.

To improve on that, we’d need to hook into psyntax or something and
we’re very far from that.  :-)

But I think it’s acceptable, and probably helpful to newcomers.  What
would be problematic is if ‘remove’ would point to ‘remove’ in the “rnrs
base” section of the Guile manual, for example, but it seems we don’t
have anything like that.

Now we need more examples in the manual!

Ludo’.