Export more symbols and publish HTML documentation, with syntax highlighting

[vindarel]

Hi,

This PR does a few things:

• export more symbols: this already is super helpful to discover the library.
• fix the documentation build scripts in a couple minor ways (no "johnw", fix a sb-introspect warning…)
• produce a few more docstrings .texinfo files
  • however, the make docstrings target doesn't create the docstring file for many symbols and I don't get why. I wouldn't try more, but use another documentation build system.
• add syntax highlighting to code snippets thanks to https://github.com/orthecreedence/highlight-lisp, whose JS and CSS are included in the main branch (and ignored with the .gitattribute).

The docs/index.html can be built on a new gh-pages branch and published on GitHub, following this setting (and deleting the docs/.gitignore file which excludes it):

The result is shown here:

https://lisp-maintainers.github.io/periods/

It is possible to commit the index.html file on master too.

I could squash the commits or send a proper PR for the export of symbols.

Thanks for considering this PR, as it will help us fellow Lispers discover and use this library.

Best,

[svetlyak40wt]

Documentation looks very cool!