I've looked at a lot of existing projects and some do and some don't.

seems like a good idea

I think it is common to include at the root but there is no explicit place anything looks at

https://docs.oracle.com/javase/tutorial/deployment/jar/manifestindex.html Is there a way to document the license and its path using the jar's Manifest file?

no

the most common way to specify a license is with a Maven pom, which has some attributes for this

It seems that lein adds both a copy of the license from the project root into the META-INF folder of the jar, but also adds a license xml section into the pom.xml. Am I correct that tools.build does not provide any such feature?

it does the same

or rather it makes a pom, and can sync from a template pom.xml if you provide one, which can include license info

I see, so that would add the pom xml, but not the actual license.

and you would need to copy the license file in