tools: add unified plugin changing links for html docs

This commit introduces additional stage in the process of generating
html docs from markdown files. Plugin transforms links to *.md files
in the respository to links to *.html files in the online documentation.

Fixes: https://github.com/nodejs/node/issues/28689

PR-URL: https://github.com/nodejs/node/pull/29946


Reviewed-By: Anna Henningsen <anna@addaleax.net>