[chore] Pin otel-specification hyperlinks

[pellared]

Why

Hyperlinks pointing to "main" make the builds flaky. Moreover, it may be hard for the reader if the hyperlink does not work at the time of reading.

What

• In CONTRIBUTING.md point to https://opentelemetry.io/docs/reference/specification/ to make sure that the contributors use the latest version of the specification during developement
• Pin all other hyperlinks in code and docs to https://github.com/open-telemetry/opentelemetry-specification/tree/v1.20.0

[codecov]

Codecov Report

Merging #4033 (d06e246) into main (c338d8e) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4033   +/-   ##
=====================================
  Coverage   82.7%   82.7%           
=====================================
  Files        179     179           
  Lines      13498   13498           
=====================================
  Hits       11176   11176           
  Misses      2101    2101           
  Partials     221     221           

Impacted Files	Coverage Δ
exporters/otlp/otlpmetric/internal/header.go	100.0% <ø> (ø)
exporters/otlp/otlptrace/internal/header.go	100.0% <ø> (ø)
exporters/prometheus/exporter.go	83.9% <ø> (ø)
exporters/zipkin/model.go	94.7% <ø> (ø)
sdk/resource/resource.go	76.5% <ø> (ø)
sdk/metric/pipeline.go	89.6% <100.0%> (ø)