docs: move `not nil` to the experimental page (#14027)

When I heard that this feature existed, and found the 2018 changelog entry that said `not nil` was made experimental (https://github.com/nim-lang/Nim/blob/devel/changelogs/changelog_0_19_0.md#changes-affecting-backwards-compatibility), I looked for `not nil` documentation in https://nim-lang.org/docs/manual_experimental.html. When I didn’t find it there, I initially assumed the feature had no documentation. This change moves the documentation to where readers will expect it.

As well as moving the text to another file, I added instructions for enabling the experimental feature and tweaked some wording.

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