Add HDMF ↔ LinkML mapping conventions documentation

[rly]

Fixes #1487

Motivation

This adds the initial HDMF ↔ LinkML mapping conventions document, the first deliverable of the "Read and write LinkML schema for HDMF (CSRMatrix / base.yaml)" epic (#1486). Before any reader, writer, or fixtures are built, we need an agreed, written contract for how HDMF Schema Language (HDMFSL) constructs map to LinkML, so that the translation round-trips faithfully into HDMF's Spec objects. This doc is that contract.

It is scoped to the constructs used by base.yaml (Data, Container, SimpleMultiContainer) and sparse.yaml (CSRMatrix), packaged as a minimal test namespace, and explicitly marks what is out of scope so it is not mistaken for a complete mapping.

The new file is docs/source/linkml_mapping.rst, added to the "Resources" toctree in docs/source/index.rst.

How to test the behavior

cd docs
sphinx-build -b html source _build/html
# open _build/html/linkml_mapping.html

Verified: the docs build with 0 warnings, linkml_mapping.html renders, and the page appears under the Resources toctree.

Checklist

• Did you update CHANGELOG.md?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed the Contributing Guide?
• Does the PR use "Fix #XXX" notation?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.18%. Comparing base (62469df) to head (cfd3f6a).

Additional details and impacted files

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  Coverage   93.18%   93.18%           
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  Files          41       41           
  Lines       10176    10176           
  Branches     2103     2103           
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  Hits         9483     9483           
  Misses        415      415           
  Partials      278      278           

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[rly]

@jwbear This is an initial pass of a document that describes the mapping between HDMF Schema Language and LinkML. Please translate sparse.yaml from the HDMF Common namespace into LinkML following these conventions, and add the translated yaml into the doc. If anything is not clear, please ask, and then we should probably clarify the doc. Your fresh eyes looking at this doc will be invaluable is assessing whether it is clear and comprehensive. I suggest we tackle this together with pair programming this week after you are familiar with the task. We can coordinate over Slack.