[Docs IA][8.15 & 8.16] Remove troubleshooting section

[nastasha-solomon]

Fixes #5712 by removing the "Troubleshooting" section from the TOC. As part of this cleanup, I also did the following:

• Relocated images used in troubleshooting topics to their appropriate folders. For example, all of the images in the docs for troubleshooting exceptions were moved to the detections/images folder.
• Renamed the "Detection rules" troubleshooting topic to "Troubleshoot detection rules"
• Deleted the troubleshooting-intro.asciidoc file and the troubleshooting folder in our docs dir
• Removed the include of the troubleshooting-intro.asciidoc file in the main TOC's index file

Preview:

• Overall TOC
• TBD

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[nastasha-solomon]

The reason why the CI checks for this PR failed and a possible solution: #5712 (comment)

[nastasha-solomon]

run docs-build rebuild

[nastasha-solomon]

Closing this PR as we've decided to keep the Troubleshooting section.