docs: prefix absolute internal links with /xorl/ base path

[qywu]

Summary

49 in-text markdown links across 20 doc files used absolute paths like `](/server-training/...)` without the `/xorl/` base prefix. Since the site is published at `https://togethercomputer.github.io/xorl/\`, those links render as `href="/server-training/..."` and 404 on click.

Sidebar links use Starlight's `slug:` mechanism and were unaffected — this only impacts prose/table links inside page content.

Fix: prefix every `](/...)` link with `/xorl/` across `docs/src/content/docs/`.

Test plan

• Verify a previously-broken link works after deploy, e.g. from the server-training overview click the "Launching & Configuration" link in the Quick Start table
• Spot-check a few links in `moe/expert-parallelism.mdx` and `server-training/sglang.mdx`