[D&M] Remote clusters cleanup

[florent-leborgne]

^^

• There's room for merging more things together later but this does a minimal cleanup and alignment between what the various files say.
• Also moved API key tabs to display 1st because the Certificate model is deprecated
• Cleaned up references to Elasticsearch service, some became ecloud, some became ech
• I intentionally left mentions of some things that are pre-9.0 because in a cloud context, users can still run older versions of the stack, and if we remove them, they'll be documented nowhere since the cloud docs are unversionned.
• Might have to revisit applies tags once we have the final implementation in hands

Closes: https://github.com/elastic/docs-projects/issues/345

[shainaraskas]

thanks for doing this. I think it makes sense as an MVP. put an issue in to investigate further restructuring opportunities because this content is super repetitive.

couple things:

• small formatting errors
• referencing stack versions without specifying they're stack
• some stuff I don't remember now

generally I commented on the first I found, but because these pages are mirroring each other the issues exist in multiple places

[shainaraskas]

a perfect page????