docs(web-trackers): document preserveOriginalReferrer tracker option

[snowplow-claude-review]

What

Documents the new preserveOriginalReferrer tracker option added to browser-tracker-core. Three files updated:

tracker-setup/initialization-options/index.md

• Added preserveOriginalReferrer row to the initialization options table, positioned after preservePageViewIdForUrl (same preserve* family), with default false, type boolean, and a link to the new anchor.
• Added preserveOriginalReferrer: false to both the JS-tag and Browser-npm code examples so the option is discoverable at the point of setup.

tracking-events/page-views/index.md

• Added ### Preserve the original referrer in SPAs subsection under the existing 'Change ID behavior for SPAs' section.
• Explains the freeze behavior, the no-op for empty document.referrer (direct navigation), and includes code examples for both tracker variants.

tracking-events/index.md

• Expanded the 'For Single Page Apps' tip callout to present two approaches side by side: the declarative preserveOriginalReferrer: true (recommended for most integrations) and the existing imperative setReferrerUrl(document.referrer) (retained for cases needing more control). All existing content preserved.

Reviewer checklist

• Table row format and column order in initialization-options/index.md matches the preservePageViewIdForUrl row.
• The new ### Preserve the original referrer in SPAs heading in page-views/index.md follows a </Tabs> block without a transition sentence — minor style gap noted; not blocking but worth a one-liner fix next time the section is touched.
• All internal anchor links verified by path search.

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Draft PR opened for review — please verify and run CI before merging.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
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