[DOCS] Reorder some lines in the class diagram (part 1)

[oliverklee]

This is in preparation for #1298.

[coveralls]

coverage: 57.935%. remained the same
when pulling 1cccce6 on docs/reorder-1
into e69e8ca on main.

[JakeQZ]

I think the comments ("start/end of generated part") should be removed with this change, since they no longer apply. (Ultimately it will all be generated, so they will become unnecessary anyway.)

[oliverklee]

I think the comments ("start/end of generated part") should be removed with this change, since they no longer apply. (Ultimately it will all be generated, so they will become unnecessary anyway.)

I've temporarily removed them in #1307.

When we autogenerate the whole diagram again, we'll need to have those markers (or something similar) back, as we need to have something for the generation and insertion script to recognize which part to replace.