DEV/DOC: update vendored code pages for `_lib.decorator`

[lucascolley]

looks like this is missing from scipy.github.io/devdocs/dev/core-dev/index.html#vendored-code and https://github.com/scipy/scipy/issues/21232 (and perhaps other places in tools that control checks)

https://github.com/scipy/scipy/pull/23119#issuecomment-2948554960

See https://github.com/scipy/scipy/pull/23125

[lucascolley]

I updated the tracker, docs pages still need doing

[ev-br]

docs pages still need doing

This PR or a follow-up?

[lucascolley]

this is an issue :)

[lucascolley]

To address this issue:

• update https://github.com/scipy/scipy/blob/main/doc/source/dev/core-dev/vendored-code.rst.inc with an entry for scipy/_lib/decorator.py and https://github.com/micheles/decorator to match https://github.com/scipy/scipy/issues/21232