docs: fix broken localhost links and "Datasream" typo in manifests

[nightcityblade]

Fix two families of user-facing text issues:

1. Broken localhost links (5 files): Replace http://localhost:8000/guide/providers.html#kubernetes-provider with the public docs URL https://www.elastic.co/guide/en/fleet/current/kubernetes-provider.html in Istio and Swimlane manifest descriptions.

2. "Datasream" typo (3 files): Fix Datasream Dataset name → Data stream dataset name in Prometheus manifest titles.

Closes #17969

[cla-checker-service]

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[elastic-vault-github-plugin-prod]

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[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[botelastic]

Hi! We just realized that we haven't looked into this PR in a while. We're sorry! We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!

[nightcityblade]

Still relevant 👍 The patch is intentionally narrow, just fixing the broken localhost references and the manifest typo. Happy to adjust if the team wants it bundled differently, but as-is it should still be safe to merge.