Update feed to support RSS, Atom, and JSON

[splorp]

Adapt these updated snippets from the kirby3-feed plugin.

https://github.com/bnomei/kirby3-feed/tree/master/snippets/feed

[splorp]

Related to this, the current ->xml() field helper should be replaced with a ![CDATA[]] wrapper around the description field content in snippets/latest.php

<description><![CDATA[<?= $page->text()->kirbytext() ?>]]</description>

This might not be an issue if using the updated snippets basically as is.

[splorp]

RSS feed updated per commit ab7546d