Identify orphan pages

**Operation System**: `all`

**DocFX Version Used**: 2.41

**Template used**: `n/a`

**Steps to Reproduce**:
n/a `feature-request`

**Expected Behavior**:
It would be good to have an option to list orphan pages. These are those that are not in any TOC nor referred in any other MD file.

It is completely fine that these pages are generated as today as these may be hidden, directly linked ones, but it would be really good to have a tool to check if a file is intentionally not reachable through navigation on the generated site.

**Actual Behavior**:
n/a `feature-request`