[Feature Request] Option to generate PDF Table of Contents from document headings

## Summary

Add a configuration option to generate the PDF Table of Contents page from actual markdown headings within documents, rather than exclusively from `toc.yml`.

## Motivation

Currently, the PDF TOC page is always generated from `toc.yml`. While `toc.yml` is useful for site navigation structure, it often doesn't reflect the detailed structure within individual documents. For single-document or small-document-set PDFs, a TOC generated from markdown headings (`#`, `##`, `###`, etc.) would be more useful and require less manual maintenance.

## Proposed Solution

Add a configuration option in `toc.yml` or `docfx.json` to control the PDF TOC source:

```yaml
# In toc.yml
pdfTocSource: headings # or 'toc' (default)
```

Or alternatively:

```json
// In docfx.json globalMetadata or pdf section
"pdfTocSource": "headings"
```

### Possible values:
- `toc` (default): Current behavior, TOC from `toc.yml`
- `headings`: Generate TOC from `#`/`##`/`###` headings in markdown files
- `both`: Merge `toc.yml` structure with in-document headings

### Additional options to consider:
- `pdfTocHeadingDepth`: Maximum heading level to include (e.g., `3` for `###`)

## Use Case

I maintain technical documentation where each markdown file is a self-contained chapter. The `toc.yml` provides top-level navigation, but when generating a PDF, I want the TOC to show the internal structure of each document (sections, subsections) which are defined by headings in the markdown, not duplicated in `toc.yml`.

## Related Issues

- #5490 — Discusses PDF bookmarks/outline (which can use headings via `WkDefaultOutline`), but not the TOC page
- #9343 — Chromium-based PDF implementation, TOC page still uses `toc.yml`
- #1882 — PDF feature requests, assumes `toc.yml`-based TOC

No implementation files, tests, or entry points are named. Start by locating the existing PDF TOC generation and toc.yml handling, then review issues #5490 and #9343; done means a documented configuration selects the TOC source and tests cover headings, default toc behavior, and any supported merge or depth options.

Written by the indexing model from the issue text.