Add note about implications of using too many Selectize

As suggested by @fredrikekelund we should add a note in Learn that brings to attention the fact that on a large scale, too many Selectize can slow down the page load dramatically.

Issue Reference: https://github.com/getgrav/grav-plugin-admin/issues/736

Start by locating the relevant Learn page and review the referenced Grav admin issue about Selectize. Add a note explaining that using too many Selectize instances at large scale can dramatically slow page loading; the work is done when the warning appears in the appropriate documentation context.

Written by the indexing model from the issue text.