tools: build all.json by combining generated JSON

Notes:

1) Removed a number of root properties that did not seem relevant:
   source, desc, and introduced_in.  There no longer is a source, and
   the other two are from the first include and do not reflect the
   entire API.

2) As with https://github.com/nodejs/node/issues/20100, the current
   "desc" properties sometimes contained in-page links, other times
   referenced another page, and often did not match the links in the
   original HTML or JSON file. I chose to standardize on external links
   as "desc" values are isolated snippets as opposed to all.html which
   can be viewed as a standalone and self contained document.

3) Eliminated preprocessing for @include entirely, including the test
   case for this function.

4) _toc.md was renamed to index.md.

5) index comments no longer appear in embedded TOCs (left hand side
   column in the generated documentation.

PR-URL: https://github.com/nodejs/node/pull/21637


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>