Update API.md - Add route part & ToC

[sufuf3]

Related issue: #154

[codecov-io]

Codecov Report

Merging #178 into develop will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop    #178      +/-   ##
==========================================
+ Coverage    77.71%   77.8%   +0.09%     
==========================================
  Files           45      45              
  Lines         2217    2217              
==========================================
+ Hits          1723    1725       +2     
+ Misses         394     393       -1     
+ Partials       100      99       -1

Impacted Files	Coverage Δ
src/prometheuscontroller/expression.go	85.88% <0%> (+0.61%)	⬆️

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