Black is invisible

[pp-mo]

There was no whatsnew for #3518 to publicise what's been done.
I think it's worth it just to inform people that the code format testing has been updated.

Meanwhile, even if you go looking in the developers' guide, the new information doesn't appear for some odd reason.

So this line says that the new "code_format.rst" page should now appear on the devguide menu, but here in the built docs it mysteriously does not.

I couldn't work out what's wrong here, but if we get a fix to give this some publicity, then obviously we'll fix that too...

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Acceptance Criteria:

• code_format.html appears in the developer's guide in the "latest" documentation (fixed in Fix problems with export and echo command. #3577 and Revert to single-line command for doctr invocation. #3578)
• Add whats new docs_change to mention code_format.html page (see Added whatsnew for Black. #3581)

[bjlittle]

@pp-mo Yeah, I noticed that the latest docs weren't rendering any changes. Oddness that definitely needs investigating 😕

[pp-mo]

Since #3577 , still not working.

I think I messed up the doctr command.
Please see #3578

[lbdreyer]

@pp-mo assuming #3578 works, it looks like all that's left for this ticket is a whats new for black