Add examples-and-data getting-started page and landing index

[brendancol]

Closes #3249

This is the last of the six getting-started PRs from #3249. The other five already merged (#3251, #3252, #3257, #3258, #3284), leaving the examples/data page and the landing-page rewrite.

• Add getting_started/examples_and_data.rst: documents the xrspatial CLI (examples, copy-examples, fetch-data, clean-data), where downloads land (xrspatial-examples/), --force / --path, and how to bring your own data instead.
• Rewrite getting_started/index.rst from a bare toctree into a short landing page that orients new users across the four pages and points onward to the user guide and API reference. Adds examples_and_data to the toctree.

Docs only. No code, no public API, no backend behavior changed.

Test plan

• Verified the CLI surface against python -m xrspatial --help (commands and download folder name).
• Checked every :doc: cross-reference resolves to an existing page.
• Built the docs with sphinx-build -b html and confirmed no new warnings for the changed pages.

[brendancol]

PR Review: Add examples-and-data getting-started page and landing index

This is a docs-only PR (two reStructuredText files), so the correctness, backend-parity, and performance sections of the review template do not apply. I focused on whether the documented commands are accurate and whether the Sphinx cross-references resolve.

Blockers

None.

Suggestions

• examples_and_data.rst: "The data they read sits in the data/ folder next to them" matches the pyct fetch-data layout (its --help says datasets are placed in path/data), but not every example notebook reads from a data/ subfolder. If you want to be precise, say the data lands in xrspatial-examples/data/ rather than implying every notebook reads from there. Not blocking.

Nits

None.

What looks good

• The CLI surface matches python -m xrspatial --help exactly: examples, copy-examples, fetch-data, clean-data, the xrspatial-examples download folder, and the --force / --path flags.
• Every :doc: cross-reference (user_guide/index, reference/geotiff, reference/index, quickstart, installation) resolves to an existing page.
• reST title underlines are the right length.
• The pip install pyct fallback matches the import guard in xrspatial/__main__.py.
• The index.rst rewrite keeps the toctree (now hidden), so navigation is unchanged while adding a readable landing page.

Checklist

• Algorithm / backend / NaN / dask / benchmark items: not applicable (docs only).
• Cross-references resolve.
• CLI claims verified against the installed command.
• sphinx-build -b html reads both pages with no new warnings.

[brendancol]

Follow-up review (after c72f839)

The one suggestion from the first pass is addressed: the examples page now states that downloaded datasets land in xrspatial-examples/data/ instead of implying every notebook reads from a data/ subfolder.

No remaining findings. Still docs-only; CLI claims and cross-references re-checked and accurate.