Updating JSON-RPC API with info_get_reward method

[ACStone-MTS]

What does this PR fix/introduce?

This PR introduces the newly added info_get_reward method to the JSON-RPC API documentation.

Additional context

The JSON-RPC will require additional updates, but this will get necessary information out for third parties.

Checklist

(Delete any that aren't relevant)

• Docs are successfully building - yarn install && yarn run build.
• 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.
• My changes follow the Casper docs style guidelines.
• All technical procedures have been tested (if you want help with this, mention it in Reviewers).

Reviewers

@ipopescu

[ipopescu]

Looks good!