[editorial] Spec updates for OTel website publication

[chalin]

• Closes #148
• Contributes to https://github.com/open-telemetry/opentelemetry.io/issues/2927
• Minor copyedits only to accompany some of the changes made per the task list below

Updates (task list taken from #148) made in this PR:

• [ ] Demote all headings one level, except for the page title
• [ ] Code quote all example.com references
• [ ] Add Hugo front matter
• [ ] 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.
• [ ] Replace links:
  • [ ] Outdated: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/otlp.md -> https://opentelemetry.io/docs/specs/otlp/

Local repo preview: https://github.com/open-telemetry/opamp-spec/blob/9ba5f927bec431e8997d0cdc6e406bd82740cd31/specification.md

/cc @tigrannajaryan @svrnm @cartermp

[chalin]

Anything left to do before this can get merged?