docs(transformations): add OneLake external data sources guide

[Khyat-Cognite]

Summary

Slice 3 of 3 — splits PR #2713.

Documentation only. Depends on PR #2714 (data classes) and PR #2715 (client API) being merged first so autosummary targets exist.

What's in this PR (~150 lines)

• docs/source/transformations_external_data.rst — quickstart: register OneLake source, verify usability, run transform with ext_onelake(), list/delete
• docs/source/transformations.rst — toctree link to the dedicated page (no duplicate automodule for external_data, avoiding Sphinx duplicate-object warnings)

What came before

PR	Scope
1 — Data model (#2714)	ExternalDataSource*, OneLake*, IAM capability, data-class unit tests
2 — Client API (#2715)	client.transformations.external_data_sources, sync codegen, API + integration tests

What comes next

Nothing — this completes the feature. After all three PRs merge to master, close the monolithic PR #2713.

Test plan

• build_docs CI green (linkcheck + no duplicate-object warnings)

Parent

Replaces the docs portion of #2713.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.66%. Comparing base (cbe7e16) to head (812236a).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           feat/onelake-edc-api    #2716      +/-   ##
========================================================
- Coverage                 93.68%   93.66%   -0.02%     
========================================================
  Files                       510      510              
  Lines                     51578    51578              
========================================================
- Hits                      48322    48312      -10     
- Misses                     3256     3266      +10     

see 5 files with indirect coverage changes

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