chore: refactor code

[ubogdan]

Describe the PR
Refactor code

• remove duplication
• use embed to store generated doc templates
• remove code for go<1.18

Relation issue
None

Additional context
None

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: +0.68 🎉

Comparison is base (7394a48) 77.11% compared to head (afae73c) 77.80%.

❗ Current head afae73c differs from pull request most recent head f666794. Consider uploading reports for the commit f666794 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #1539      +/-   ##
==========================================
+ Coverage   77.11%   77.80%   +0.68%     
==========================================
  Files          23       21       -2     
  Lines        5479     5415      -64     
==========================================
- Hits         4225     4213      -12     
+ Misses       1080     1026      -54     
- Partials      174      176       +2     

Impacted Files	Coverage Δ
gen/gen.go	70.68% <27.27%> (-16.46%)	⬇️
parser.go	91.78% <100.00%> (ø)
utils.go	100.00% <100.00%> (ø)

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