docs: Replace godoc.org with pkg.go.dev

[alexandear]

The PR replaces outdated godoc.org with pkg.go.dev.

See golang/go#49212 for details about the deprecated godoc.

The badge is created at https://pkg.go.dev/badge/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.01%. Comparing base (2b8c7fa) to head (f23b0d9).
Report is 130 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3301 +/- ##
==========================================
- Coverage 97.72% 93.01% -4.72% 
==========================================
Files 153 172 +19 Lines 13390 11777 -1613 ==========================================
- Hits 13085 10954 -2131 - Misses 215 729 +514 - Partials 90 94 +4 

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[gmlewis]

Thank you, @alexandear !
LGTM.
Merging.