Add guide for getting the most out of Bits Investigations

[dgreen15]

Summary

• Adds a new page, "Getting the Most Out of Bits Investigations," covering how to tune Bits Investigation accuracy: bits.md, selecting the right monitors, runbooks, Skills, external integrations (Confluence, GitHub, third-party tools), and feedback/memories, plus how to verify changes land using Bits Chat.
• Documents Skills as a fourth knowledge source on the Knowledge Sources page.
• Trims the Integrations and Settings page title from "Configure Integrations and Settings" to "Integrations and Settings," and updates the Features nextlinks on the section index accordingly.

Test plan

• Preview build renders the new page correctly under /bits_ai/bits_investigation/
• All internal links and anchors resolve
• External links (Confluence, GitHub, Actions > Skills, filtered Supported Monitors URLs) are correct

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/dgreen15/add-bits-investigation-accuracy-guide/bits_ai/bits_investigation/getting_the_most_out

Modified Files

• https://docs-staging.datadoghq.com/dgreen15/add-bits-investigation-accuracy-guide/bits_ai/bits_investigation/
• https://docs-staging.datadoghq.com/dgreen15/add-bits-investigation-accuracy-guide/bits_ai/bits_investigation/configure
• https://docs-staging.datadoghq.com/dgreen15/add-bits-investigation-accuracy-guide/bits_ai/bits_investigation/knowledge_sources

[buraizu]

Created DOCS-15592 for the editorial review.