805 - Add Sidecar documentation in the Condor branch

[ipopescu]

What does this PR fix/introduce?

This PR adds information on the Casper Sidecar and refreshes related content for operators and developers. It also covers the merge of 3 event streams into one. Most of the content added lives here:

• source/docs/casper/operators/setup/casper-sidecar.md
• source/docs/casper/operators/setup/node-events.md
• source/docs/casper/developers/dapps/monitor-and-consume-events.md

To avoid duplication and maintenance issues, I added links to the casper-sidecar repository in Github, instead of duplicating the content in the docs.

Open TODOs:

• Once the .deb package is published, update the installation command and sample output in operators/setup/casper-sidecar.md.
• Currently, all links point to the Sidecar's feat-2.0 branch. Once this branch merges, we'll need to update the links in the docs.

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.
• [x] All technical procedures have been tested (if you want help with this, mention it in Reviewers).

Reviewers

@ACStoneCL