[docs] make dump-rectors command open to public

[TomasVotruba]

Closes #3208

How to Generate your own Rector Overview in Markdown?

E.g. AllRectorsOverview.md

# prints markdown to the console output
vendor/bin/rector dump-rectors 

# prints markdown to the file
vendor/bin/rector dump-rectors > all_rectors.md

# prints rules found in specific directory - good for your own sets
vendor/bin/rector dump-rectors my_project/rector/src/ > all_rectors.md