Fix gatsby build pipeline

[janbrasna]

1. Replaces current CI workflow (broken) with newer build actions.
2. Obsoletes old gh-pages deploy path writing to branch by instead publishing the build artifacts directly to the (existing) github-pages environment:
   • once working, old gh-pages branch can be removed as it's not used anymore;
   • the github-pages deployment exists from the old implicit runs, no need to create…
   • … however it will be most likely protected to only allow gh-pages to publish, needs to be changed to master in repo settings (or removed altogether) [docs]
   • no CNAME is needed since the value is configured in repo settings — should be still present from the legacy Pages so just needs checking. [docs]
3. For now uses Node 14 as it won't build on 16/20, GH encourages to upgrade to at least to Node 16 so this might need looking into eventually, all the deps are rather old.
4. Downgrades Gatsby and SASS back before dependabot broken the env in #193 and #156 💣 🙉
5. Adds missing fbjs dep that should've been loaded by some react-dom deps however was missing 🤷
6. Removes gh-pages dep (as publishing from local dev is not needed anymore), incl. the run commands.
7. Removes some unnecessary files, updates the JSX content where URLs are hard-coded, adds Node compatibility to readme.

[janbrasna]

@noahsark769 You can see the workflow run details.

It's rather slow and emits loads of warnings. but seems to work ok otherwise. You probably know better if all the warnings are present the whole time locally too.

As this won't be run too often I don't care that much about output sizes, caches and 2-3mins build time as long as these legacy versions work, run and build;) so you can just forget about deploys and have them always published on master. 🚀