Add missing index.md for FRED CMP docs

[quaggaone]

Description

The subsequent pages weren't showing up in the navigation because of the missing index.md

Affected versions

It is affecting both 2.x and 3.x

Relevant issues

n/a

I noticed this issue because the subsequent pages were showing up on the separate FRED Docs here: https://modxcms.github.io/fred/cmp-elements.html

Are these separate FRED Docs synced with the Docs here in some way or are they maintained separately from each other?

[Mark-H]

Are these separate FRED Docs synced with the Docs here in some way or are they maintained separately from each other?

They are separate.