Add Programming model documentation

[MathiasMagnus]

This PR succeeds #3318 and builds on top of #3405, adds a single commit on top the doc reorg.

[gandryey]

I think we should add the depth for Multi Grid picture. It's multiple grids, thus it should show the depth of gray rectangles called "Grid"

[MKKnorr]

This is already portraied by having several Grids in the picture. The grids don't necessarily have to be of uniform size, so laying them on top of each other could be misleading

[MKKnorr]

I took over this PR and fixed all the remarks I still had on the original PR, in a hopefully sensible manner, and will create new issues for the docs team for the rest

@jujiang-del please review and merge in Gerrit when appropriate

[jujiang-del]

Merging to Gerrit
https://gerrit-git.amd.com/c/compute/ec/hip/+/1034056

[MKKnorr]

Closing, as this has been upstreamed via gerrit. Thanks!