docs: Improve formatting in README

[alexandear]

The PR improves formatting in README.md:

• replace outdated godoc.org with pkg.go.dev;
• set text link for raw URLs;
• replace whitespace with TAB in Go example code;
• add missing ### for some titles;

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.05%. Comparing base (2b8c7fa) to head (90cb24e).
Report is 133 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3300 +/- ##
==========================================
- Coverage 97.72% 93.05% -4.67% 
==========================================
Files 153 172 +19 Lines 13390 11772 -1618 ==========================================
- Hits 13085 10955 -2130 - Misses 215 723 +508 - Partials 90 94 +4 

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[gmlewis]

Thank you, @alexandear !
Just a few nits, please, then we should be ready to merge.

[gmlewis]

Thank you, @alexandear !
LGTM.
Merging.