Creates "Connect to Google vertex" llm connector guide

[benironside]

Fixes #5459 and fixes #5373. Creates a new LLM connector guide for Google Vertex AI.

Preview: Google Vertex AI

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @benironside? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticdocs]

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Issues? Visit #next-docs in Slack

[elasticdocs]

🚀 Built elastic-dot-co-docs-preview-docs successfully!

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• Built with commit 50eb0dc

Issues? Visit #next-docs in Slack

[Charelzard]

Please see comments listed.

[natasha-moore-elastic]

Not sure why, but all videos on the page render as screenshots.

[benironside]

They render correctly if you refresh, in my experience 🤷

[mergify]

This pull request is now in conflicts. Could you fix it @benironside? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b 5459-connect-to-vertex-llm-guide upstream/5459-connect-to-vertex-llm-guide
git merge upstream/main
git push upstream 5459-connect-to-vertex-llm-guide