Encourage html-safe API in layouts/rendering guide

[oreoshake]

This is a second pass at #30576 /cc @georgeclaghorn

Summary

While the code example was not unsafe, it encourages the use of confusingly unsafe APIs
(specifically html_safe). We have a safe alternative and we should encourage people to use
it.

I don't know if there is an open issue for this, but a quick search doesn't reveal one.

Other Information

Finally, if your pull request affects documentation or any non-code
changes, guidelines for those changes are available
here

I've added:

composed with html_safe-aware APIs`

I'm not sure if the language is OK or whether or not it should link to https://github.com/rails/rails/blob/b9ecb5797b97ba557a70efd66d2ff2c22501723f/guides/source/active_support_core_extensions.md#output-safety for more details.

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