page: add `fields` attribute to article tags

[petecooper]

URLs

• Back: https://github.com/textpattern/textpattern.github.io/blob/master/tags/article.md https://github.com/textpattern/textpattern.github.io/blob/master/tags/article_custom.md
• Front: https://docs.textpattern.com/tags/article https://docs.textpattern.com/tags/article_custom

Explanation of immediate task

Add fields attribute documentation.

Resources to support effort

• https://forum.textpattern.com/viewtopic.php?pid=332980#p332980

Current to-dos

• assign author
• write words
• close issue