"Add Data from Splunk" feature deprecation

[benironside]

Fixes elastic/docs-content-internal/issues/403 by removing the Add Data from Splunk page, which was deprecated in 9.0 This was a technical preview feature, and the current docs are no longer accurate; the functionality is no longer present starting from 9.0 per @jamiehynds. Also removes links to that page, since we shouldn't be directing users to a deprecated feature.

After the removal of this page, https://www.elastic.co/docs/solutions/observability/get-started/other-tutorials/ became a pretty small stub section. There was only one tutorial in the section, so I opted to remove the section and bring the remaining tutorial from that section up a level in the ToC. This necessitated a variety of edits to the redirects file.

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/get-started/quickstarts.md
• solutions/security/get-started/ingest-data-to-elastic-security.md
• solutions/observability/get-started/tutorial-monitor-java-application.md

[florent-leborgne]

I think I would prefer to keep the "Other Observability tutorials" section even with only one item in there because it keeps the structure of pages clean and doesn't imply any sequence between pages.

Would like more opinions on this especially from the folks who revisited this section recently to make sure that this structural change doesn't contradict with principles from the last rewrite or future plans. CC @theletterf @mdbirnstiehl

[theletterf]

Thanks for this PR, @benironside! I agree with Florent, let's keep the "Other observability tutorials" category for now. I'm assessing if the Java tutorial could be removed, in which case I'd take care of removing the category, too.

[benironside]

Closing this since a lot of the changes won't be needed in light of Florent's and Fabri's feedback. Remaining work to take place in #3808