Stop docs builds from importing example Dags to highlight them

[amoghrajesh]

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Sphinx's ModuleAnalyzer.for_module() locates a module's source by importing it. The examples rendered by the exampleinclude directive live in providertests/system trees that are not / should not be importable during a docs build.

Noticed this error earlier today on #70960

Module "system.ibm.mq.example_dag_message_queue_trigger" could not be loaded.
Full source will not be available.
...
  File "/opt/airflow/devel-common/src/sphinx_exts/exampleinclude.py", line 150, in register_source
    analyzer = ModuleAnalyzer.for_module(modname)
...
ModuleNotFoundError: No module named 'system.ibm'

The import buys nothing here: doctree_read already knows the file on disk, and Sphinx offers for_file() to tokenize it directly.

Ref: https://github.com/sphinx-doc/sphinx/blob/b04a2101295ac3fb725b16111eda0284b6da4cca/sphinx/pycode/__init__.py#L89-L103

Tested it works fine after doc build:

---

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• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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[ashb]

Sounds sensible if it works. Thanks!

[amoghrajesh]

Alright rebased, lets see how it goes.

[potiuk]

Yeah... if works. :)

[amoghrajesh]

Guess it does. Merging

[github-actions]

Hi maintainer, this PR was merged without a milestone set.
We've automatically set the milestone to Airflow 3.3.3 based on: backport label targeting v3-3-test
If this milestone is not correct, please update it to the appropriate milestone.

This comment was generated by Milestone Tag Assistant.

[github-actions]

Backport successfully created: v3-3-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-3-test