[editorial] Spec updates for OTel website publication

[chalin]

• Closes [editorial] Update spec to make it suitable for publication on the OTel website #148
• Contributes to OpAMP spec adjustments opentelemetry.io#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  • Outdated: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/otlp.md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]

@tigrannajaryan et al. - since there is no significant content above this point, I'd drop this header. It naturally follows that the prose below the page title is introductory. E.g., there is no "Introduction" heading in the OTLP spec. WDYT?

[chalin]

Anything left to do before this can get merged?