Adds the ufunc class to the documentation so that it is included
in the generated html docs and all links to it work.

Slightly modify ufunc docstring to remove unrecognized heading
name.

Describing ufunc copy behaviour when input and output overlaps.

Closes #16749

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* DOC: Fixes for 18 broken links

This, with PR #16465, should fix nearly all the remaining broken links
on the site. 4 or 5 others should be easy to fix and just
need attention from someone more knowledgeable -- will
open an issue. For release notes with dead links,
I could usually find links on archive.org for roughly contemporary
versions.

* DOC: Update to "Fixes for 18 broken links #16472"

* Obsolete links, previously commented out, now deleted:
https://github.com/numpy/numpy/pull/16472#discussion_r433928958

* Semantic markup for reference to Python class:
https://github.com/numpy/numpy/pull/16472#discussion_r433553928

* Missing :ref: in internal link:
https://github.com/numpy/numpy/pull/16472#discussion_r433554484

Not included: Resolution on using external/internal doc link in .py:
https://github.com/numpy/numpy/pull/16472#discussion_r433554824

* DOC: Add internal link for 'Fixes for 18 broken links' PR #16472

Making reference [1] an internal link in function_base.py => numpy.vectorize.html

* DOC: Redirect 2 link fixes in PR #16472

* governance.rst link reverted
* ununcs.rst `overridden` link goes where it was meant to


per https://github.com/numpy/numpy/pull/16472#pullrequestreview-424666070

According to the docs, the list can be of lengths 1, 2, or 3, but only
length 3 appears to work:

```
>>> import numpy as np
>>> np.sin(1, extobj=[16, 1, lambda: None])
0.8414709848078965
>>> np.sin(1, extobj=[16, 1])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: UFUNC_PYVALS must be a length 3 list.
>>> np.sin(1, extobj=[16])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: UFUNC_PYVALS must be a length 3 list.
```

See #15214

API: Forbid Q<->m safe casting

That is, uint64 <-> timedelta64.

Allowing this casting was weird, because Q<->q is forbidden, yet q and m are both int64.

This fixes gh-12927