Fix: Updated/Added details around APIs and include quickstarts

[adammarshallNR]

Added link to the Synthetics Quickstarts library on github pages Updated Broken Link and Certificate Check API types Added rows for Nerdgraph, Terraform under additional features, noted REST API deprecation

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[akristen]

Hi @adammarshallNR ! I've added @bpeckNR as a reviewer to ensure that we're all in line about updating this doc. I want to explain some of my changes:

• I went ahead and removed the videos in this doc, since we find that videos aren't enormously helpful. They were also both around 2 years old and we provide the same information by linking out to other docs or outright explaining the processes.
• I updated the verbiage around our synthetics quickstarts. Since these are open source, we want to make sure that customers understand they are community-sourced and not supported by New Relic.
• I also fixed some broken links in the doc as well.

Once Brian approves these changes, I might take the opportunity to give this doc a freshness update and soften out the edges. Thanks a bunch for filing this PR!

[bpeckNR]

I like the call out about these examples being open source and not supported by New Relic. It looks like the sentence about the examples has an extra "view examples of" at the end. Maybe we should change it to something like this:

These examples are open source and not supported by New Relic, but contain valuable scripts and snippets that may help you cover common use cases in scripted API and scripted browser monitors.

[akristen]

I like it! i'll add this in