docs(examples): Conditional dev dependencies with dependency groups

[arrdem]

Case study requested in Bazel slack demonstrating conditional dev-dependency
inclusion using PEP 735 dependency groups, a Bazel string_flag, and select().

The example shows a py_dev_binary wrapper macro that includes dev-only
packages (ipdb, pytest) by default and strips them out in release builds
(bazel build //:app --config=release).

See examples/dev_deps/README.md for the full walkthrough.

Changes are visible to end-users: no

Test plan

• Manual testing; please provide instructions so we can reproduce:

cd examples/dev_deps
bazel build //:app              # dev mode — ipdb/pytest included
bazel build //:app --config=release  # prod mode — ipdb/pytest excluded

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

87 tests (100.0%) were fully cached saving 45s.

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Bazel 9 (Test)

All tests were cache hits

87 tests (100.0%) were fully cached saving 1m 14s.

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Bazel 8 (Test)

e2e

All tests were cache hits

25 tests (100.0%) were fully cached saving 13s.

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Bazel 9 (Test)

e2e

All tests were cache hits

25 tests (100.0%) were fully cached saving 17s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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