[CI] Move away from the use of Gulp

[chalin]

Roll our own scripts indepentent of Gulp, even for (file) stream processing.

Here are the tasks to roll over:

$ npx gulp --tasks
Tasks for ~/tmp/opentelemetry.io/gulpfile.js
├── default            Display usage instructions
├── lint-md            Check markdownlint rules. For details, use --info.
├── prune              Prune --num <n> entries from static/refcache.json file. For details, use --info.
└── validate-registry

I had originally thought that use of Gulp would help in making our use of markdownlint more efficient. It has, but we can do stream processing in other ways (be if to use OS commands to drive paralleism, or JS-native functionality). Besides, because of the current setup, lint-md is a bit more of a hack; plus it's fix functionality doesn't fully work. So given that I'm going to be rewriting how we use markdownlint (including our own custom rules), it feels like it's a good time to move drop Gulp, the overhead (in learning and maintenance), isn't worth it.

WDYT @svrnm (since you wrote one of the tasks) et all?

[svrnm]

Yeah. 👍 when I wrote my task I only plugged it into gulp because it was there already. I would not mind replacing it