docs: add instructions for running the documentation site locally

[cuixq]

This PR updates docs/README.md to provide clearer instructions for contributors to run and test the documentation site locally in docker or natively.

To run documentation site in docker, this PR introduces docs/docs.Dockerfile to allow consistent local documentation serving without requiring a local Ruby environment.

[another-rex]

Can you add the dockerfile in osv-scanner, and add that to the instructions? I've always had trouble building directly with bundler.

[cuixq]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces instructions and a Dockerfile for running the documentation site locally. The changes to docs/README.md add sections for both Docker-based and native local development, and the new docs/docs.Dockerfile is intended to provide a consistent environment.

My review focuses on improving the new Docker setup. I've identified a critical issue in the Dockerfile where an invalid base image is used, which will prevent it from building. I have also suggested improvements to the README.md instructions to clarify the required working directory and to enhance the docker run command for a better developer experience with live-reloading.

[another-rex]

We probably can just remove the .bundle copy part.

Though @cuixq can you test if you can actually run the docs locally? I seem to be getting conflicts on unable to resolve the dependencies

[cuixq]

@another-rex it seems due to the ruby version e.g. commonmarker 0.23.12 has requirement >= 2.6, < 4.0.

I have modified the docker image to be using ruby:3 for now. We probably need to do this for osv-scanner too.