Export content test examples as JSON

[MdSaifAliMolla]

closes #1772

[gnprice]

As I wrote at #1918 (comment) last week:

Thanks. It looks like you have a couple of other PRs open already:

• Get route-transition duration robustly,instead of hard-coding #1920
• KaTeX overline underline border #1919

Let's start with feedback on those. After you've successfully revised those to be highly reviewable PRs (https://zulip.readthedocs.io/en/latest/contributing/reviewable-prs.html), then you'll be more prepared to come back and do the same with this one.

It's a significant amount of work for someone to review a PR. For the PR to be a useful contribution to Zulip, you need to do the work to make it easier to review — otherwise, it's less work for one of us to just write the change ourselves than to review your PR.

It's OK that you don't start out having all the skills to make a PR highly reviewable. We spend a significant part of our time teaching new contributors those skills (which you'll find useful for a wide range of software projects in the future). But please don't keep opening new PRs while you have existing PRs where you're getting feedback on how to do that.

Instead, if you'd like to contribute to Zulip, two good ways to spend your time are:

• Revise those existing PRs, as discussed above. When you feel like you don't see how to improve those, then…
• Read the codebase and our Git commits where we've developed it. See our short guide to exploring the codebase (which in fact is an approach I find valuable for just about any codebase). This will help you develop your sense of how the code works, and also of how we write it and how we present our changes.