Delete legacy `embeddable_api` plugin

[lukeelmers]

Part of #51560

Currently src/legacy/core_plugins/embeddable_api is basically doing nothing besides re-exporting the static public contract of the embeddable plugin. We should delete it to clean things up.

• remove core_plugins/embeddable_api
• update static imports referencing embeddable_api to pull from the new platform plugin
• add Dev Docs note to PR to make sure 3rd party devs who may have been depending on that plugin are aware it has been relocated

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)