Add dokka setup for Kotlin API docs

[thunderbiscuit]

This PR adds the required setup for Dokka 2.0, allowing us to build the API documentation for bdk-jvm and bdk-android. Note that the README files are what ends up on the homepage/landing page of the API docs website. You can build and test for yourself by running

cd bdk-jvm/
just build-docs

And open the lib/build/dokka/html/index.html file using a small live server like the one provided in vscode, or the built-in python one which should serve the website at http://localhost:8000/:

cd lib/build/dokka/html/
python -m http.server 8000

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[reez]

ACK 04279c0

Thanks again for making it easy as a reviewer to follow the steps to test! I always really appreciate that.