Establish v2 spec nightly metadata on GitHub Pages deployment

[agriyakhetarpal]

• Adds a script to add v2 metadata for nightly cross-build environments in a JSON file (see generated JSON file)
• The publish.yml workflow file now pushes a commit to main. I chose to not use a PR because it adds a bit of churn, but I can change the approach if asked
• Added a pages.yml workflow to deploy the file on GitHub Pages

There is no v1 file here, just v2, because it's the first time we are doing it and it doesn't make sense to go with the old spec.

I will follow this up by putting together a PR in pyodide-build to use this metadata for those who would like to use nightly and nightly-debug builds.

xref pyodide/pyodide#6248, pyodide/pyodide-build#349

[agriyakhetarpal]

Okay, all green – I'm merging to try this on GitHub Pages, and I'll open PRs if I have to make follow-up fixes! I'll leave the Pyodide and pyodide-build PRs open as I've requested reviews from you on those, @ryanking13. This is for the nightly environments and thus has no impact, as there's no downstream adoption.