[PyTorch Debug] Custom feature tutorial.

[pggPL]

Description

This PR adds short custom feature tutorial to precision debug tools docs.

Type of change

• [x] Documentation change (change only to the documentation, either a fix or a new content)

• [ ] Bug fix (non-breaking change which fixes an issue)

• [ ] New feature (non-breaking change which adds functionality)

• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

• [ ] Infra/Build change

• [ ] Code refactoring

• [x] I have read and followed the contributing guidelines

• [x] The functionality is complete

• [x] I have commented my code, particularly in hard-to-understand areas

• [x] I have made corresponding changes to the documentation

• [x] My changes generate no new warnings

• [x] I have added tests that prove my fix is effective or that my feature works

• [x] New and existing unit tests pass locally with my changes

[pggPL]

It's better to debug this PR with docs generated, instead of looking into changes.