feat: add BSD and macos platforms to --platform option

[owenvoke]

Closes #234 and closes #235

[acuteenvy]

This doesn't work, because macos is not a directory. tldr --platform macos <page_name> always prints "documentation is not available".

There's also android, which isn't on the list here.

[owenvoke]

Ah, so we need to actually alias it. Will mark this as draft and work on the required changes.

I'll add Android as well. I feel like it's a bit redundant duplicating the platforms here though, so perhaps we can extract to a variable or something.

[kbdharun]

Hi, any updates on this?