docs: port remark-base-links off deprecated markdown.remarkPlugins (Astro 7)

Context

#46 upgraded the docs site to Astro 7 (via @nebari/starlight v0.2.0). Astro 7 ships a new native markdown pipeline and now emits a deprecation warning on build:

`markdown.remarkPlugins`, `markdown.rehypePlugins`, and `markdown.remarkRehype` are deprecated. Pass them to `unified({...})` from `@astrojs/markdown-remark` directly instead.

The site's custom remark-base-links plugin (docs/site/src/plugins/remark-base-links.js) is wired through markdown.remarkPlugins in docs/site/astro.config.mjs. It prefixes root-absolute links and images with the deployed base path (/nebari-frames/).

Current state

It still works. The deprecated option continues to run because @astrojs/markdown-remark is present as a peer of both Astro and Starlight. This is a future-proofing cleanup, not a bug.

What to do

Port remark-base-links off the deprecated option before a future Astro major removes it. Per the Astro v7 upgrade guide there are a few paths:

• Switch to the unified() processor via @astrojs/markdown-remark (declare it as a direct dependency of docs/site), keeping the existing remark plugin, or
• Port the plugin to the new native MDAST/HAST plugin API.

Acceptance

• Docs build produces no markdown.remarkPlugins deprecation warning.
• Base-path link rewriting still verified in the built HTML (root-absolute content links prefixed with /nebari-frames/).

Start with docs/site/astro.config.mjs and docs/site/src/plugins/remark-base-links.js, then review the Astro v7 upgrade guidance for @astrojs/markdown-remark. Run the docs build to confirm the deprecated markdown.remarkPlugins warning is gone. Verify the built HTML still prefixes root-absolute content links and images with /nebari-frames/.