Prepare for spec 1.21.0, including OTLP and semconv being factored out

[chalin]

Prep for next release of the specification:

• Normalize all site page links re. the new location of the spec
  • Normalize paths to spec pages #2703
  • Go:
    • [website_docs] Update path to spec, run formatter opentelemetry-go#4084
    • Update opentelemetry-go submodule with normalized spec paths #2722
  • OTel spec: fix links into GitHub like View Page Source #2704
• [editorial] specification/README: adjust Hugo front-matter linkTitle field opentelemetry-specification#3481 - drop temporary edits to adjust script
• [editorial] Add config README and aliases for relocated pages opentelemetry-specification#3499
• Spec: link to supplementary guidelines in repo, update refcache and more #2723
• Update OTel specification, drop workarounds, fix links #2724
  • Drop temporary ignore rule from link-checker config

Later:

• Adjust to spec being split up:
  • Adjust to move of OTLP specs into opentelemetry-proto #2642
  • Adjust to move Semantic Conventions out of OTel spec #2721
• [editorial] Normalize links in semconv/http-metrics opentelemetry-specification#3498 for equivalent

[chalin]

Done. Closing. Will track #2642 and #2721 separately.