fix(docs): link to reference docs section on googleapis.dev

[bcoe]

No description provided.

[codecov]

Codecov Report

Merging #139 into master will decrease coverage by 2.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
- Coverage    99.3%   96.85%   -2.45%     
==========================================
  Files           4        4              
  Lines         572      572              
  Branches      120       96      -24     
==========================================
- Hits          568      554      -14     
- Misses          4       17      +13     
- Partials        0        1       +1

Impacted Files	Coverage Δ
src/gaxios.ts	93.92% <0%> (-4.86%)	⬇️
src/index.ts	97.36% <0%> (-2.64%)	⬇️
src/retry.ts	98.63% <0%> (-0.69%)	⬇️

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