Reformat snapshot lifecycle policy API docs

[jrodewig]

Updates the snapshot lifecycle policy API documentation to match the Elastic API reference template.

Relates to elastic/docs#937 and #45195

• Put Snapshot Lifecycle Policy API ([DOCS] Reformat put snapshot lifecycle policy API docs #47811)
• Get Snapshot Lifecycle Policy API ([DOCS] Reformat get snapshot lifecycle policy API docs #47827)
• Execute Snapshot Lifecycle Policy API ([DOCS] Reformat docs for several snapshot lifecycle policy APIs #47998)
• Get Snapshot Lifecycle Stats API ([DOCS] Reformat docs for several snapshot lifecycle policy APIs #47998)
• Delete Snapshot Lifecycle Policy API ([DOCS] Reformat docs for several snapshot lifecycle policy APIs #47998)
• Execute Snapshot Lifecycle Retention API ([DOCS] Reformat docs for several snapshot lifecycle policy APIs #47998)

We are only backporting these changes to 7.4.

Branch + file containing working changes:
https://github.com/jrodewig/elasticsearch/blob/reformat.slm-apis/docs/reference/ilm/apis/slm-api.asciidoc

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-distributed

[dakrone]

We are only backporting these changes to 7.x and 7.3.

SLM is only in the 7.x branch (7.4+), so no need to backport to 7.3

[elasticmachine]

Pinging @elastic/es-core-features

[jrodewig]

Thanks @dakrone. I've amended the original description.

[wchrisdean]

[doc issue triage]