Test all examples from library-user-guide & user-guide docs

[ugoa]

Which issue does this PR close?

• Closes Run / Test all examples in Documentation #14435

Rationale for this change

What changes are included in this PR?

Library User Guide:

• docs/source/library-user-guide/adding-udfs.md
• docs/source/library-user-guide/api-health.md
• docs/source/library-user-guide/building-logical-plans.md
• docs/source/library-user-guide/catalogs.md
• docs/source/library-user-guide/custom-table-providers.md
• docs/source/library-user-guide/extending-operators.md
• docs/source/library-user-guide/extensions.md
• docs/source/library-user-guide/index.md
• docs/source/library-user-guide/profiling.md
• docs/source/library-user-guide/query-optimizer.md
• docs/source/library-user-guide/using-the-dataframe-api.md
• docs/source/library-user-guide/using-the-sql-api.md
• docs/source/library-user-guide/working-with-exprs.md

User Guide:

• docs/source/user-guide/example-usage.md
• docs/source/user-guide/faq.md
• docs/source/user-guide/introduction.md
• docs/source/user-guide/cli/index.rst
• docs/source/user-guide/cli/overview.md
• docs/source/user-guide/cli/datasources.md
• docs/source/user-guide/cli/usage.md
• docs/source/user-guide/cli/installation.md
• docs/source/user-guide/configs.md
• docs/source/user-guide/concepts-readings-events.md
• docs/source/user-guide/dataframe.md
• docs/source/user-guide/explain-usage.md
• docs/source/user-guide/expressions.md
• docs/source/user-guide/crate-configuration.md
• docs/source/user-guide/sql/sql_status.md
• docs/source/user-guide/sql/index.rst
• docs/source/user-guide/sql/scalar_functions.md
• docs/source/user-guide/sql/write_options.md
• docs/source/user-guide/sql/explain.md
• docs/source/user-guide/sql/information_schema.md
• docs/source/user-guide/sql/select.md
• docs/source/user-guide/sql/special_functions.md
• docs/source/user-guide/sql/dml.md
• docs/source/user-guide/sql/operators.md
• docs/source/user-guide/sql/ddl.md
• docs/source/user-guide/sql/aggregate_functions.md
• docs/source/user-guide/sql/data_types.md
• docs/source/user-guide/sql/subqueries.md
• docs/source/user-guide/sql/window_functions.md

Are these changes tested?

Yes

Are there any user-facing changes?

No

[ugoa]

take

[ugoa]

So to address #14557, I managed to strip off the setup code with some regex trick in Python docs/rustdoc_trim.py, and now the doc shows clean snippets while the doc-test still able to pass.
A few points to consider:

• It is a simple implementation, but I think it should be good enough to cover the requirement.
• For now it only pre-processes the doc dir docs/source/library-user-guide, as for the user-guide/, it can be in next PR

[alamb]

Looks like this PR ended up with a change to datafusion-testing too -- I'll try and remove that shortly

[alamb]

Close/reopent o start CI