OTel intro rework

[chalin]

• Copyedit "What is OTel" - Rework of What-is-OTel and Instrumentation pages #6487
• Add video link to what-is-opentelemetry page #6550
  • Add "What is OTel?" video to "What is OTel?" documentation page #6474
  • Also add links to training in What next / Learn more
• I think that we should move the "What next" list from "What is OTel" to the main docs landing page, and edit front page "Learn more" button to point to the Docs landing page, move the video etc.

Extra(?):

• Fix the main diagram in https://opentelemetry.io/docs/ (maybe). For details, see comment below.

[chalin]

@open-telemetry/docs-approvers @LisaHJung @open-telemetry/sig-end-user-approvers - shouldn't "Frontends" in the right-most block of the following diagram be "Backends"?

Btw, this is from https://opentelemetry.io/docs/.

[danielgblanco]

I assume you mean "shouldn't "Frontends" in the right-most block of the following diagram be "Backends"?". I agree. I think the word "frontend" here was probably used to refer to their UI/alerting layer, but we name them "backends" everywhere else.

[chalin]

Oops, yes, I meant "right-most". Fixed, thanks! 😅