docs: Use secure URLs in string literals

[alexandear]

This PR replaces http:// with https:// where possible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (f46ce36) to head (6d0641d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3657   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files         185      185           
  Lines       16378    16378           
=======================================
  Hits        14945    14945           
  Misses       1249     1249           
  Partials      184      184           

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[gmlewis]

Some of these URLs like "postbin.org" look suspicious.
How do we know we aren't spreading malware through these links?

[alexandear]

PostBin is a legitimate service for developers

But I can replace it with localhost.

[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.