Add inheritance graphs to docs

[dstansby]

Fixes #169. Here's what it looks like rendered locally for me:

[samcunliffe]

Yeah, do you think it's worthwhile?
Since #168 there is now some relatively interesting class structure.

Also: custom plot users will need to inherit from... SingleAxisWidget now, right?

[dstansby]

I think it's worth it, especially if #167 happens and it gets even more interesting

[samcunliffe]

Do we need @ruaridhg because: co-authored commit?

[dstansby]

I think I hit the merge buton, it's just waiting for tests.

[dstansby]

@samcunliffe can you try giving this a fresh approve?

[ruaridhg]

Diagram looks good!