Docs: Set up for new Logstash 8.x branch

[karenzone]

Logstash tech writer: @karenzone
Docs release manager: @lcawl

Logstash release docs

• Cut branches:
  • logstash
  • logstash-docs
  • ingest-docs
    • Update backport tooling for ingest-docs: Add 8.x branch to backport and mergify configs ingest-docs#1311
• Update plugin docs: updated docs for 8.x logstash-docs#1724