Improve custom theme documentation guide

[ErraticFox]

Documentation Feedback

I've been using Angular for over a year now and the page for custom component theming either needs removed or redone. It causes a lot more confusion that it has ever helped and the example is quite case specific and you simply can do a Google search to see there are absolutely tons of confusion about this. Just look at this answer on Stack Overflow, it's a prime example. It's not simple, not well documented, and Googling my problems usually are just leads to work around because of the lack of documentation. I'd like to help the documentation, but anyone hardly know how it works.

Affected documentation page: https://material.angular.io/guide/theming-your-components

[jelbourn]

Fixed in #22268

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