Re: SB-MANUAL contrib prototype

I posted the first version of sb-manual here hoping that no one would
cry for the indices that were cruelly removed. Predictably, someone
confessed in private to having printed the SBCL manual at least on one
occaison. It must be admitted that in print, those indices actually
help. I previously added back the "Function and Macro Index" and other
similar indices, but not the Concept index, which is quite unlike the
others.

I finally figured out how to add back support for concept indexing. Now
all those removed @cindex lines can be added back but in a nicer way.
This is a proof of concept, with only four lines in the SBCL Concept
Index. Direct link in HTML:

https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.html#concept-index

The rest of the Texinfo-generated stuff:

> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.html
> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.info
> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.info-1
> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.info-2
> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.info-3
> https://quotenil.com/blog-files/sbcl-pax-doc/texinfo/sbcl.pdf

Ironically for one being so linky, PAX didn't support indices at all.
Now it does: proper tracking of all links, multi-level index keys,
sort-as, configurable categorization of referrers (all on the
https://github.com/melisgl/sbcl/tree/pax-doc branch for now). Direct
link to the Indices section in HTML:

https://quotenil.com/blog-files/sbcl-pax-doc/html/sbcl-manual.html#x-28-221-22-20MGL-PAX-3A-3ADYN-29

It's a proper index contentwise, unlike the Texinfo version. It's easy
if the document actually has the links (in the Texinfo we don't, so we
only index the definitions for now).

By default, plain text and Markdown don't get indices, so only the PDF
has it (see below). It's pretty similar to the HTML.

> https://quotenil.com/blog-files/sbcl-pax-doc/sbcl-manual.txt
> https://quotenil.com/blog-files/sbcl-pax-doc/sbcl-manual.md
> https://quotenil.com/blog-files/sbcl-pax-doc/sbcl-manual.pdf
> https://quotenil.com/blog-files/sbcl-pax-doc/html/sbcl-manual.html