Documentation for the feature flag "headless.cookieDomainPerSite"

[Digi92]

Is your feature request related to a problem? Please describe.
he feature flag "headless.cookieDomainPerSite" exists in the codebase but is not documented.
The documentation mentions a middleware as a possible solution but does not reference the existing implementation.

Describe the solution you'd like

• Mention the feature flag "headless.cookieDomainPerSite" in the docs: https://docs.typo3.org/p/friendsoftypo3/headless/main/en-us/Configuration/Index.html#feature-flags
• Reference the existing middleware here: https://docs.typo3.org/p/friendsoftypo3/headless/main/en-us/Configuration/Preview.html
• I think a cross-reference to Nuxt-TYPO3 regarding proxy headers proxyReqHeaders: ['cookie'] would be nice: Unclear Documentation regarding Preview of Hidden Pages #684 (comment) -> https://t3headless.macopedia.io/nuxt-typo3/guide/headers/#proxy-request-headers-to-typo3

Best regards
Digi92

[tmotyl]

Thanks for the hint. Woudl you be so kind to prepare a PR for this change?