Update the learning back button to use our new landing pages

[Eric-Arellano]

Every _toc.json in Learning sets parentUrl, which sets up the back-left arrow in the left ToC.

documentation/learning/courses/quantum-chem-with-vqe/_toc.json

Line 2 in 90bdf5e

"parentUrl": "/learning",

Now that we're launching the new pages /learning/courses and /learning/modules, we should change these parentUrl values to be more specific rather than just /learning.

Be careful with merging this into main because we need to ensure that the new pages are actually live.