Update dissect and user_agent readme

[oeyh]

Description

Update dissect and user_agent readme to refer to documentation website.

Issues Resolved

Part of #2740

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[graytaylor0]

Just saw build is failure you can run this to fix the checkstyle error

 * What went wrong:
Execution failed for task ':data-prepper-plugins:user-agent-processor:spotlessMarkdownCheck'.
> The following files had format violations:

      README.md
          @@ -1,4 +1,4 @@
           #·User·Agent·Processor
           This·processor·parses·User-Agent·(UA)·string·in·an·event·and·add·the·parsing·result·to·the·event.
           
          -See·the·[`user_agent`·processor·documentation](https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/processors/user-agent/).
          +See·the·[`user_agent`·processor·documentation](https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/processors/user-agent/).
  Run './gradlew :data-prepper-plugins:user-agent-processor:spotlessApply' to fix these violations.