[Doc] Fix build from source and installation link in README.md

[Yikun]

This patch:

• fixes wrong links in readme to avoid the 404.
• replace vllm.readthedocs.io to docs.vllm.ai to avoid useless rediect.

Related: #11935

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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 do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[Yikun]

cc @DarkLight1337 who involved in #11935

[mgoin]

LGTM, thanks for fixing the broken links!

[DarkLight1337]

Oh, I forgot about those links, thanks for fixing!