Fix/move the generic-arithmetic docs

[cgay]

https://opendylan.org/documentation/library-reference/language-extensions/numbers.html#the-generic-arithmetic-library needs a lot of love.

• Much of it hasn't been fully updated to Sphinx markup
• It specifically says that the big-integers library provides a 64-bit implementation of <integer>. That's only correct on 32-bit machines. I assume it's 128 bits on 64-bit machines.
• It's hard to find. It's in the library reference, but not under "big-integer" or "generic-arithmetic" or "dylan (extensions)" but under Dylan Language Extensions > Integers. I propose to break the actual libraries in Dylan Language Extensions out into their own library documents so they show up in the list of libraries, and reference them with links instead. This should make them more accessible and visible.

[cgay]

I agree with 2021 me.

[cgay]

I agree with 2022 me.

[cgay]

• 665841a750afc10f385c77a6d03de50d1ec918f5
• da43d1e64b64be2c29a5a94e441851c328d28824

I think only the third bullet point in the initial comment hasn't been addressed.