Interface stability documentation

[MrAlias]

• Update the project versioning policy to call out interfaces can be extended.
• Update project contributing documentation with information on how and when interfaces can be extended.
• Add documentation to exported information about their stability or developer comments stating they cannot be changed.

Resolves #1714

[codecov]

Codecov Report

Merging #2012 (37b9eb7) into main (39fe809) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2012   +/-   ##
=====================================
  Coverage   73.0%   73.0%           
=====================================
  Files        171     171           
  Lines       8687    8687           
=====================================
+ Hits        6346    6348    +2     
+ Misses      2072    2070    -2     
  Partials     269     269           

Impacted Files	Coverage Δ
propagation/propagation.go	78.5% <ø> (ø)
sdk/resource/auto.go	87.5% <ø> (ø)
sdk/trace/id_generator.go	100.0% <ø> (ø)
sdk/trace/sampling.go	100.0% <ø> (ø)
sdk/trace/span.go	88.9% <ø> (ø)
trace/trace.go	98.2% <ø> (ø)
exporters/jaeger/jaeger.go	94.2% <0.0%> (+1.1%)	⬆️