chore: Rename 'anchors' to index

[bstasyszyn]

closes #961

Signed-off-by: Bob Stasyszyn Bob.Stasyszyn@securekey.com

[codecov]

Codecov Report

Merging #962 (abb0fbc) into main (5f9bded) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #962      +/-   ##
==========================================
- Coverage   89.42%   89.35%   -0.08%     
==========================================
  Files         159      159              
  Lines       15132    15132              
==========================================
- Hits        13532    13521      -11     
- Misses        976      985       +9     
- Partials      624      626       +2     

Impacted Files	Coverage Δ
...ctivitypub/service/activityhandler/inboxhandler.go	85.32% <100.00%> (ø)
pkg/activitypub/vocab/anchoreventtype.go	90.99% <100.00%> (ø)
pkg/activitypub/vocab/options.go	97.94% <100.00%> (ø)
pkg/anchor/anchorevent/anchorevent.go	81.35% <100.00%> (ø)
pkg/anchor/handler/proof/handler.go	84.34% <100.00%> (ø)
pkg/anchor/writer/writer.go	91.58% <100.00%> (ø)
...pub/service/inbox/httpsubscriber/httpsubscriber.go	90.12% <0.00%> (-9.88%)	⬇️
pkg/activitypub/service/monitoring/monitoring.go	77.35% <0.00%> (-2.84%)	⬇️
pkg/observer/pubsub.go	94.49% <0.00%> (ø)

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