Added Public CasperLabs JSON-RPC Endpoints

[dylanireland]

What does this PR fix/introduce?

This PR adds the public CasperLabs JSON-RPC endpoints as the preferred method of communicating with the network.

Additional context

The current method encourages choosing from a list of currently online Casper Network peers, many of which may have RPC ports disabled or could theoretically act in malicious ways.

Checklist

• [x] Docs are successfully building - yarn install && yarn run build.
• [x] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• [x] All external links have been verified with yarn run check:externals.
• [x] My changes follow the Casper docs style guidelines.

[github-actions[bot]]

🚀 Deployed on https://668da4fed87737f11de528a3--sensational-creponne-f0e56d.netlify.app

[ipopescu]

Merging on behalf of @dylanireland.