Skip doc generation if no plugin version up.

[mashhurs]

Description

Issue: #45
Currently, reference doc generator generates and rewrites the docs. Although there is no plugin version release, there might be a content change. For this case, we usually revert back to original version to avoid doc breakings.

Example cases:

• https://github.com/elastic/logstash-docs/pull/983/commits
• https://github.com/elastic/logstash-docs/pull/990/commits

Closes: #45

This PR introduces the skip-existing option for the reference doc generator. When applying this option results no-override existing docs.

Test

• This logic will be consolidated into existing Reference Doc Generation workflow.
• On my local, changed the contents without touching version numbers, executed following command:
  bundle exec ruby plugindocs.rb --skip-existing --output-path ../logstash-docs ../logstash/plugins_version_docs.json
  As a result, no version bumped plugins are skipped.
• Executed the forked repo workflow and as a result doc update PR was created.

[kares]

LGTM, couple minor suggestions left in-line

also given how smart --skip-existing is to check for :version: 🤔 if this is a behavior one would always want and thus have it on by default? (no strong feelings here but if anyone ever needs to run the tool manually he/she/it should end up with sensible defaults).