How to include manual documentation in forge doc --serve?

[a-t-0]

Thank you for your marvellous work on providing such a fast and well-documented framework for Solidity development! How can one include manual documentation in forge doc?

I tried adding a manual dir docs/manual/hello.md with content:

# hello
some text.

and adding the manual dir in the src line of the book.toml like:

[book]
src = ["src", "manual"]  # Include both "src" and "manual" directories
title = ""

[output.html]
no-section-label = true
additional-js = ["solidity.min.js"]
additional-css = ["book.css"]
git-repository-url = "https://github.com/TruCol/Decentralised-Saas-Investment-Protocol"

[output.html.fold]
enable = true

However, when I run forge doc --serve, this book.toml is overwritten back to:

[book]
src = "src"
title = ""

[output.html]
no-section-label = true
additional-js = ["solidity.min.js"]
additional-css = ["book.css"]
git-repository-url = "https://github.com/TruCol/Decentralised-Saas-Investment-Protocol"

[output.html.fold]
enable = true

The docs/manual/hello.md and content is preserved however. Hence, I would like to kindly ask:

How can one embed manual documentation when auto-generating the documentation with forge doc?