Add docs for cdk/coercion

[mateusrcm]

Just a question and a suggestion, why such things as cdk/coercion doen't have documentation on your website, and why don't u guys implemented it yet?

[jelbourn]

We originally wrote docs for the subpackages that we considered most useful. At that time, cdk/coercion only had two functions in it (coerceBooleanProperty and coerceNumberProperty). It's grown a bit since then and probably does warrant it's own docs now.

[umairhm]

Since this documentation was missing, I thought to write some reference docs around this with examples. You can find it here:
https://indepth.dev/angular-cdk-coercion/

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