DOC-2608: Add an RSS feed for the TinyMCE Changelog

[FarzadHayat]

Ticket: DOC-2608

RSS Feed: http://docs-hotfix-7-doc-2608.staging.tiny.cloud/docs/tinymce/7/rss.xml

RSS Validator: https://validator.w3.org/feed/
RSS Viewer: https://rssviewer.app/
RSS Spec: https://www.rssboard.org/rss-specification

Changes:

• Worked on this in my own time. Thought it would be a cool feature to have.
• Add Antora extension to tinymce/7 to generate rss.xml during build process
• Automatically updates when changelog.adoc is modified
• Error handling mechanism to ensure build process continues if the extension fails for any reason
• You can test it out by installing an RSS Reader on your smart phone and entering the RSS URL

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• [-] modules/ROOT/nav.adoc has been updated (if applicable).
• [-] Included a release note entry for any New product features.
• [-] If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc.

Review:

• Documentation Team Lead has reviewed

[LewisAtTiny]

I've spent the morning trying to understand RSS feeds and this is actually really cool! Good job Farzad!