Missing plugin VPR docs between placeholder generation and last commit date.

### Description
We generated and added `logstash-integration-logstash` plugin placeholder to the repo. After that, when we generated VPR we witnessed that workflow couldn't catch es-output changes. The reason behind, when we generated VPR, we get the last commit date (`git log -1 --date=short --pretty=format:%cd`) and seek the new published gems after that date.

### Solutions
1) place a caution to encourage running VPR before on placeholder generator
2) when generating placeholder, also catch the VPR changes or abort by saying that it breaks the process
3) add an ability to generate VPR docs for individual plugin

Option 2 is better from operation point of view.