Switching to grafana URLs

[olegbespalov]

What?

This PR switches the URLs from the k6.io/docs/ to the grafana.com/docs/k6/latest/

Why?

grafana.com/docs/k6/latest/ is the primary source of the documentation right now.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f35e679) 72.95% compared to head (8f21347) 72.90%.

❗ Current head 8f21347 differs from pull request most recent head d7c8ecf. Consider uploading reports for the commit d7c8ecf to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3543      +/-   ##
==========================================
- Coverage   72.95%   72.90%   -0.05%     
==========================================
  Files         280      278       -2     
  Lines       20938    20936       -2     
==========================================
- Hits        15275    15264      -11     
- Misses       4693     4698       +5     
- Partials      970      974       +4     

Flag	Coverage Δ
macos	?
ubuntu	72.90% <100.00%> (+<0.01%)	⬆️
windows	?

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[codebien]

LGTM