doc: Link to the repository and official docs in doc.go

[tonyo]

At https://pkg.go.dev/github.com/getsentry/sentry-go they already show the README.md from the repository, and what we've had in doc.go (displayed under "Documentation") has been partially outdated.

To avoid these drifts in the future, let's just link to the repository and official docs in doc.go.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.07 🎉

Comparison is base (2c4f1c1) 79.83% compared to head (ff32395) 79.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
+ Coverage   79.83%   79.91%   +0.07%     
==========================================
  Files          38       38              
  Lines        3873     3873              
==========================================
+ Hits         3092     3095       +3     
+ Misses        673      671       -2     
+ Partials      108      107       -1     

see 1 file with indirect coverage changes

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