Plugin doc generation: preserve content in logstash-docs if no version bump

[karenzone]

"Destination Wins"

A plugin version bump triggers the docgen scripts to pick up doc changes for inclusion in a PR. If there isn't a version bump, the script should not pick up doc content. When it does, I often have to back out the changes to pass CI and avoid breaking the docs build.

Examples:

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

The second commit backs out changes that introduce regressions.

[karenzone]

@jsvd Here's the issue we discussed. Shall I assign it to you?

[karenzone]

I put up #49 to [hopefully] address the log4j regressions. This issue is still worthwhile, but not quite as urgent.