[CI] Remove duplicated docs build from buildkite

[hmellor]

• This job provides no additional signal that the Read the Docs build does not
• Removing it will save a small amount of money by reducing the time spent on CPU machines in buildkite CI

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request removes a duplicated documentation build step from the Buildkite CI pipeline to reduce CI costs. The changes are logical and consistent across the pipeline configuration, Dockerfile, and a test script. My review includes one suggestion for the Dockerfile to further optimize the CI process by consolidating RUN commands, which aligns with the PR's goal of improving efficiency.

[gemini-code-assist]

To optimize the Docker image and align with best practices, it's recommended to chain related shell commands into a single RUN instruction. Each RUN command creates a new layer in the Docker image. Reducing the number of layers can lead to smaller image sizes and potentially faster build times, which is consistent with the goal of this PR to reduce CI costs. Using mv vllm src/ is also more idiomatic for moving a directory into another.

RUN mkdir src && mv vllm src/