DOC: MAINT: Add NEP 43 links to NEP 42

[bjnath]

Now that NEP 43 is merged, adds the missing NEP 43 links to NEP 42.

[mattip]

Could these be

:ref:`NEP 43`

instead of

:ref:`NEP 43 <NEP43>`

[bjnath]

That doesn't work ("undefined label").

The link can be written :ref:`NEP43` (without the space) but then the NEP title is included. We don't want the title every time, just "NEP 43", which is the reason for the current coding.

OTOH, for the note at the top of the page, including the NEP titles seems a good idea, so I made that change.

[mattip]

We probably had the same discussion before, thanks for reminding me 👍 The rendered page looks good.

[mattip]

Thanks @bjnath