Add Firehose redirects

[alaudazzi]

Adds redirects for the Firehose pages removed with elastic/docs#2970.

Relates to #3749

[github-actions]

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[mergify]

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

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[alaudazzi]

@elastic/obs-docs a couple of questions:

• How do we handle the redirect for the file docs/en/aws-firehose/index.asciidoc?
• After the redirects are merged, do we need to remove the entire aws-firehose folder (asciidoc files+ images)?

[bmorelli25]

@alaudazzi unfortunately this won't work. This PR adds redirects from the Observability Guide to the Observability Guide. What we need are redirects from the Amazon Kinesis Data Firehose Ingest Guide to the Observability Guide.

Redirect files only apply to the book they are included in. Because the Amazon Kinesis Data Firehose Ingest Guide was removed in elastic/docs#2970, we can't add a redirect file to it and thus can't use Asciidoctor redirects.

I expect the web team redirects to go live any day now. Here's the issue: https://github.com/elastic/website-development/issues/12832.