Doc: Standard pattern for GitHub link doesn't resolve correctly

[karenzone]

Page: https://www.elastic.co/docs/reference/logstash/plugins/plugins-filters-elastic_integration#_getting_help_137

The "Get Help" section is populated by an included header file that uses variables to present the GitHub path. The pattern breaks for filter-elastic_integration because this plugin doesn't live in the logstash-plugins GitHub org. This will likely require a manual override.

Description of fixes

• Add new non-standard header file
• Update filter-elstic_integration file to use new non-standard header

Fixes needed

• Generated output in logstash-docs:
  • LSR
    • main: Add header for non-standard repo logstash-docs#1840
    • 9.0: Add header for non-standard repo 9.0 logstash-docs#1848
  • VPR: VPR: Use non-standard header for intg filter logstash-docs#1850
• MD files in logstash-docs-md:
  • LSR: Doc: Fix link to filter-elastic_integration github repo logstash-docs-md#18
  • VPR: Doc: Fix link to filter-elastic_integration github repo logstash-docs-md#18
• Doc source in logstash-filter-elastic_integration: