[Doc] [1/N] Reorganize Getting Started section

[DarkLight1337]

• The installation guide for each hardware platform is now nested under Installation to conserve space in the navigation tab.
  • Renamed basic installation guide to CUDA to be more explicit about the hardware platform.
  • Renamed CPU installation guide to x86 CPU in order to distinguish it from ARM CPUs.
• Renamed Debugging Tips to Troubleshooting.
• Moved FAQ from Usage section to Getting Started for better visibility.
• Replaced some relative links with project references to avoid issues when moving pages in later PRs.

[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.

🚀

[DarkLight1337]

I have already set up the redirects, feel free to merge once you approve it.