Add description for programmatic Groovy parser creation

[CCFenner]

Describe how to create a Groovy parser with a Groovy script, e.g in the script console.

related to JENKINS-52237

[codecov]

Codecov Report

Merging #118 into master will increase coverage by 1.47%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #118      +/-   ##
============================================
+ Coverage     80.69%   82.17%   +1.47%     
+ Complexity     1660     1551     -109     
============================================
  Files           244      244              
  Lines          5733     5289     -444     
  Branches        427      403      -24     
============================================
- Hits           4626     4346     -280     
+ Misses          952      792     -160     
+ Partials        155      151       -4

Impacted Files	Coverage Δ	Complexity Δ
...ns/plugins/analysis/core/steps/IssuesRecorder.java	82.01% <0%> (-0.26%)	74% <0%> (-52%)
...s/plugins/analysis/core/steps/IssuesPublisher.java	100% <0%> (+0.92%)	24% <0%> (-15%)	⬇️
...plugins/analysis/core/steps/PublishIssuesStep.java	52.14% <0%> (+2.34%)	20% <0%> (-17%)	⬇️
.../plugins/analysis/core/steps/RecordIssuesStep.java	50.6% <0%> (+2.41%)	31% <0%> (-25%)	⬇️

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[uhafner]

Thanks!