Clarify canonical documentation between guides and example READMEs (pruning)

[danielkorzekwa]

ModelOpt documentation is split between docs/source/guides/ and examples/, often with overlapping content. For example, pruning is documented in both.

examples/pruning/ and docs/source/guides/3_pruning.rst.

As a reader, I am confused which one to read and use. They both overlap and do not seem to be in sync.

[hiSandog]

The pruning docs need a single canonical path, otherwise example READMEs and guides will keep diverging. A small index table could map each pruning workflow to the maintained guide, the example README, and the supported release/version. It would also be worth adding a docs check that flags example links pointing at superseded guide locations.

[h-guo18]

cc @shengliangxu I think this is a valid suggestions that might be worth considering during the re-architecture