[editorial] Update spec to make it suitable for publication on the OTel website

[chalin]

I'll add details soon. Creating this now so that I can have an issue number to track with.

• [x] Demote all headings one level, except for the page title
• [x] Code quote all example.com references
• [x] Add Hugo front matter
• [x] Remove Copyright notice at the end of the spec page -- the repo has a LICENSE file, and when published on the OTel website, each page has a copyright notice.
• [x] Replace links:
  • [x] Outdated: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/otlp.md -> https://opentelemetry.io/docs/specs/otlp/

/assign @chalin