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

[guidanoli]

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:

# README.md

This project is about ...

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

# 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