CI: Build GitHub Pages bundle on-the-fly

[SandroHc]

Builds all the JS and CSS files required for the demo pages in docs.yml via GitHub Actions. Removes the necessity of having the JS bundles at /docs/demos/dist/, as they will be compiled and deployed on every commit to main.

Please confirm that, under Settings > Pages > Build and deployment (https://github.com/fiduswriter/simple-datatables/settings/pages), the "Source" field is set to "GitHub Actions".

I have also checked out package-lock.json to make builds deterministic - building the library now or building it in ten months should generate the same code and dependency tree. (reverted)

[johanneswilm]

Hey, in general good, but not so sure about package-lock.json. I like the idea behind it, and a few years ago everyone added them to repos everywhere for the reason you describe. A few months later they were removed again as it turned out it just wasn't very practical and one would usually delete them manually whenever doing a new build.

[SandroHc]

Never had issues with it, but sounds like a hassle if I had. I'll revert the package-lock.json changes

[johanneswilm]

@SandroHc thanks!