[nastasha-solomon]

Relates to https://github.com/elastic/docs-content-internal/issues/750

The "latest documentation" banner on the 8.x Elastic Security prebuilt rule pages links to the docs homepage (/docs) instead of the Prebuilt detection rules reference. This PR fixes that in two parts.

v3-mapping.json: maps the two hub pages to the prebuilt rules reference:

• prebuilt-rules.html (Prebuilt rule reference)
• prebuilt-rules-downloadable-updates.html (Downloadable rule updates)

url_to_v3.rb: adds a fallback for the versioned changelog pages (prebuilt-rule-<x>-<y>-<z>-<rule>.html). There are dozens of summary pages and one page per rule per release, so listing them one by one isn't practical. For these pages, the banner now:

1. Uses the rule's existing mapping (/en/security/*/<rule>.html), if it has one, to link to that rule's 9.x page.
2. Otherwise links to the prebuilt rules reference.

Exact mappings still take priority, and pages in other books aren't affected.

Testing

I ran UrlToV3 against the updated mapping, using a stub doc with an /en/security/8.19 outdir:

Page	Banner link
prebuilt-rules.html	/docs/reference/security/prebuilt-rules
prebuilt-rules-downloadable-updates.html	/docs/reference/security/prebuilt-rules
prebuilt-rule-8-19-14-prebuilt-rules-8-19-14-summary.html	/docs/reference/security/prebuilt-rules
prebuilt-rule-8-17-1-a-scheduled-task-was-created.html	/docs/reference/security/prebuilt-rules/rules/windows/persistence_scheduled_task_creation_winlog
prebuilt-rule-8-19-14-potential-sap-netweaver-exploitation.html (rule not mapped yet)	/docs/reference/security/prebuilt-rules
prebuilt-rules-api.html (existing entry)	unchanged
An unmapped security page	/docs (unchanged)
prebuilt-rule-8-19-14-foo.html in the Kibana book	/docs (unchanged)

I didn't run make check (rspec and rubocop) locally because Docker wasn't available, so I'm relying on CI for those.

This doesn't address the other direction raised in the issue: linking from the 9.x reference back to older 8.x versions.

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