Adding 8.18 and 8.18.0 details, master not bumped as it's on 9.0.0, current still 8.17

[eedugon]

This needs review as I have some concerns due to:

• We have already master pointing to 9.0.0.
• There's a comment on ECS about keeping it in 8.17.

Comments added in the code.

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[eedugon]

@lcawl , @bmorelli25 , I'd need an approval on this PR soon.
Based on @karenzone comment it looks this is blocking other docs repositories to create things for 8.18, like https://github.com/elastic/logstash-docs/actions/runs/13249296075.

The only remaining issue on this PR is if the prev-major-last: value on master.asciidoc needs to be bumped to 8.18 or if we leave it in 8.17. I've updated it to 8.18.

[bmorelli25]

run docs-build rebuild

[bmorelli25]

LGTM

[eedugon]

run docs-build rebuild

[eedugon]

@elasticmachine run docs-build rebuild