[Bug] PDF page number is not starts with `1` if cover/toc pages exists

**Describe the bug**
PDF page number is not starts with `1` when cover or toc pages are exists.

**To Reproduce**
https://dotnet.github.io/docfx/seed/articles/seed.pdf

**Expected behavior**
Page number starts with `1`

**Context (please complete the following information):**
- OS: Windows
- Docfx version: [e.g. 2.78.1]

Start by reproducing the linked seed.pdf from the DocFX documentation site and trace the PDF page-number generation entry point. Confirm how cover and table-of-contents pages affect numbering; done means the first numbered content page displays 1.

Written by the indexing model from the issue text.