Add documentation generation badges

[daveverwer]

Fixes #1887

The intention here is that the build should show the badge if it even attempted a documentation build so we can find the correct build to start debugging.

I'm not super happy with this wording:

If successful, this build generated package documentation.

I did consider:

This build attempted to build documentation.

Here's how it looks:

[finestructure]

I must be missing something – I can't figure out where the logic sits that determines which build is supposed to build the docs.

I was expecting to see somewhere something looking at the SPIManifest or to have it reported back from the build system (probably better) what build generated the docs.

[finestructure]

Oh, I see. This is the display and #1888 is the data 👍