Backporting updates to ILM org, overview, & GS (#51898)

[debadair]

• [DOCS] Align with ILM API docs ([DOCS] Align with ILM API docs #48705)

• [DOCS] Reconciled with Snapshot/Restore reorg

• [DOCS] Split off ILM overview to a separate topic. ([DOCS] Split off ILM overview to a separate topic. #51287)

• [DOCS} Split off overview to a separate topic.

• [DOCS] Incorporated feedback from @jrodewig.

• [DOCS] Edit ILM GS tutorial ([DOCS] Edit ILM GS tutorial #51513)

• [DOCS] Edit ILM GS tutorial

• [DOCS] Incorporated review feedback from @andreidan.

• [DOCS] Removed test link & fixed anchor & title.

• Update docs/reference/ilm/getting-started-ilm.asciidoc

Co-Authored-By: James Rodewig [email protected]

• Fixed glossary merge error.

Co-authored-by: James Rodewig [email protected]

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[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/ILM+SLM)