Re: SB-MANUAL contrib prototype

Since last time:

- Now that concepts are supported, I added back the concepts that were
  removed. There weren't a whole lot.

- There are no more ambiguous links in the PAX-generated output (e.g.
  EQL that would link to both the EQL type specifier and the EQL
  function in the CLHS).

- https://fixnum.com/ generates HTML, PDF, Markdown, and plain text
  versions of SB-MANUAL (and other things) every two hours.

As far as I can tell, the pax-doc branch
(https://github.com/melisgl/sbcl/tree/pax-doc) is ready, and I'll merge
it tomorrow.

Texinfo-generated output:

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