Adds some documentation improvements on opaqueness

[floriank]

In an effort to get myself hacking on gleam, I attempted working on #2111. After some back and forth and asking in Discord, I ran with https://github.com/gleam-lang/gleam/pull/2446 and just hid the const implementation for the time being.

Since I also dove a bit into CSS, this offers an explicit marker should a type be opaque. I think this should improve readability.

Potentially closes #2111.

[floriank]

Quick screenshots

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Dark mode

[floriank]

Thanks for the review - I made the changes as requested! @lpil

[lpil]

Looks like the tests are failing due to some whitespace having been added.

[floriank]

I am not too familiar with the templating here, but I've removed an extra line that was added by ...probably me fat fingering my way through vim :wink:

[floriank]

@lpil Tests run locally for me now

[lpil]

Thank you!!