tools: build all.html by combining generated HTML

Combine the toc and api contents from the generated doc/api/*.html
files. This ensures that the single page version of the documentation
exactly matches the individual pages.

PR-URL: https://github.com/nodejs/node/pull/21568
Fixes: https://github.com/nodejs/node/issues/20100


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>