docs: document OPeNDAP URL retrieval (#287)

[svsam]

Description

Closes #287.

This PR documents how to retrieve OPeNDAP service URLs from granule search results. The example filters each granule's umm.RelatedUrls entries using:

• Type == "USE SERVICE API"
• Subtype == "OPENDAP DATA"

It uses defensive .get() access and explains that collections without OPeNDAP metadata return an empty list.

This is a documentation-only change so no public APIs, library code, tests, or fixture files were changed.

Test data

Existing fixture	Granules	OPeNDAP URLs retrieved	Ordinary data URLs excluded
GPM	1	1	1
LAADS	1	1	1
Daymet	10	10	20

Verification

• pytest tests/unit/test_results.py -q -p no:cacheprovider — 16 passed
• mkdocs build --no-strict — passed
• git diff --check — passed
• Confirmed that only docs/user/howto/access-data.md changed

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📚 Documentation preview 📚: https://earthaccess--1448.org.readthedocs.build/en/1448/

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[betolink]

Thanks for this contribution @svsam ! I think it's very useful to have documentation on how to collect OPeNDAP endpoints. Approving the PR