docs(installation): clarify download/staging copy + collapse init script render

[mjohns-databricks]

Summary

Doc-only PR. Two kinds of changes to docs/docs/installation.mdx:

Clarifications (three small wording tweaks):

• "attaches three files" → "attaches three files (among others)" — the release page has more than just those three, the original phrasing implied otherwise.
• "not uploaded to the bundle Volume" → "not included in the bundle (from above)" — reads more naturally after the preceding section introduced the bundle.
• Step 3.1: name VOL_DIR as a candidate staging location for the init script itself, so readers don't have to invent a path.

Render improvement — the inline <CodeFromTest code={initScript} … /> was rendering all ~400 lines of geobrix-gdal-init.sh, dominating the install page for readers scrolling past it. Replaced with a 10-line preview + native HTML <details> disclosure containing the full script:

Preview of the first 10 lines — expand below for the full script (~400 lines).

<CodeFromTest
  code={initScript.split('\n').slice(0, 10).join('\n') + '\n# ... (expand the disclosure below for the full script)'}
  …
/>

<details>
  <summary>Show the full init script</summary>

<CodeFromTest code={initScript} … />

</details>

The MDX expression slicing happens at render time against the raw-loader string, so the preview stays in sync with the script automatically — no manual sync needed.

Test plan

• gbx:docs:dev — verify the preview renders as the first 10 lines and the disclosure is collapsed by default + expands cleanly to the full script.
• Confirm the link to [scripts/geobrix-gdal-init.sh](https://github.com/databrickslabs/geobrix/blob/main/scripts/geobrix-gdal-init.sh) in step 3 still resolves (it's unchanged but worth a glance).
• Confirm responsive behavior on a narrow viewport — <details> is native HTML so Docusaurus's default theme handles it, but worth one mobile-width sanity check.

This pull request and its description were written by Isaac.