Nested/Recursive includes

### Problem or Desire

For our new docs at work, we want to convert LaTeX to Markdown. The problem is that it has quite a lot of nested documents with `\include`, which Docsify can only go one level deep.

### Proposal

Docsify should resolve the included markdown files recursively. With protection against looping embeds.

### Additional Context

There is no need to make it a setting to preserve backwards compatibility.
First time

The issue names no files, tests, or entry points. Start by locating Docsify's handling of markdown \include directives and existing embed-resolution tests; then verify recursive inclusion, including protection against looping embeds, while preserving current behavior without adding a setting.

Written by the indexing model from the issue text.