feat: make `forge doc` generate documentation for Markdown files other than `README.md` as well

### Component

Forge

### Describe the feature you would like

Currently, `forge doc` only generates documentation for the contracts and a `README.md` file.
In some cases, however, there are other Markdown files that could be part of the `mdbook`.
For example, if the `README.md` links to other Markdown files, `forge doc` could generate documentation for those as well, and link accordingly. Example:

```markdown
# README.md

This project is about ...

[See how to contribute to the project](CONTRIBUTING.md)
```

```markdown
# CONTRIBUTING.md

You can contribute in such way ...
```

Alternatively, a list of Markdown files to generate documentation for could be listed in `foundry.toml`.

### Additional context

_No response_