60634: External Integrations: add intros + prerequisites (part 2)

[Nadia-JSch]

Continues the External Integrations UX work from #532. That PR added audience intros and prerequisites to 11 integration pages; this one covers nine further pages that had no statement of audience, outcome, or setup requirements.

Where a page already listed requirements in the wrong place, this PR moves that existing content into ## Prerequisites rather than duplicating it:

• Nagios: the Perl package install commands sat inside ## Install the Nagios Add-On after the words "there are a couple prerequisites". They now sit under ## Prerequisites, so the install heading starts at the actual install step.
• Rundeck: the Java 1.8+ requirement was a stray bullet in the installation section.
• Puppet and Chef: the **Requirements:** list had two sentences run together with no space (Python v2.7.10+Getting the Device42...). The versions moved to prerequisites and the sentence is now separate.

All prerequisites are drawn from each page's own text. No version numbers or connectivity details were invented. The Cloud Connector page gets an intro only, as it is a conceptual overview where a prerequisites list would not be accurate.

Also fixes a broken cross-reference on the PowerBI page, which told readers to meet the requirements in an "ODBC System Requirements" section that does not exist on that page. It now links to the ODBC driver page where that section actually lives.

Verified with yarn build: no broken links.

Jira issue	Description	Preview link(s)
60634	Add audience intro and prerequisites	- External Logging Prerequisites - Nagios Prerequisites - Webhooks Prerequisites - Rundeck Prerequisites - Aperture VISTA Prerequisites - PowerBI Prerequisites - Puppet and Chef Prerequisites - Foreman Prerequisites - Cloud Connector Introduction