[dev_docs] Document index management change timeline

[axw]

We've made changes to the way index management has been done over several releases, and it's become difficult to track due to the changes being spread across repositories (apm-server, integrations, elasticsearch), and debug when things go wrong.

We should document these in one place for developers to understand the timeline of changes, and then consider what's missing from user-facing release notes. e.g. https://github.com/elastic/integrations/pull/9185, apm-data plugin enabled by default in 8.15.0