Add a section stating `igniteui-theming` shouldn't be installed explicitly

[Simeon Simeonoff]

          > Where in the documentation is this?

Nice catch, we will be adding a paragraph in the docs stating that users shouldn't explicitly be installing the theming package as it is internally encapsulated in the package.

Originally posted by @simeonoff in IgniteUI/igniteui-angular#13827 (comment)