perf(build): don't build the doc-generator binaries by default

[Dandandan]

Which issue does this PR close?

Adresses: #13814

Found while profiling compile times for #24325 / #24326 / #24329 / #24330.

Rationale for this change

datafusion/core/src/bin/ holds three binaries that regenerate the docs under
docs/source/user-guide: print_config_docs, print_runtime_config_docs and
print_functions_docs. Cargo auto-discovers them and they have no
required-features, so every cargo build links all three — each one
~174MB, since each links the whole datafusion rlib.

Nothing in normal development uses them. They are run by
dev/update_config_docs.sh and dev/update_function_docs.sh, and by the CI job
that checks the committed docs are up to date.

Two places where this shows up:

Cold builds. The three binaries link after every other unit has finished,
so they sit on the critical path with nothing to overlap with.
cargo build --timings shows them occupying the last 3.5s of a
cargo build -p datafusion (~8.8s of CPU), after the last library unit
completes.

The tightest inner loop — touch a file in core, rebuild. All three are
relinked every time:

before: 3.0s  2.4s
after:  1.3s  1.1s

What changes are included in this PR?

The three binaries move behind a new non-default docs_generation feature, and
the two dev/ scripts pass --features docs_generation.

Using required-features means declaring the [[bin]] targets explicitly, since
auto-discovered targets cannot carry it.

Are these changes tested?

• cargo build -p datafusion no longer produces the three binaries
• cargo build -p datafusion --features docs_generation does
• ./dev/update_config_docs.sh still regenerates
  docs/source/user-guide/configs.md byte-identically (empty git diff
  afterwards), which is what the CI doc check compares

dev/update_function_docs.sh uses the same invocation pattern and all three of
its call sites were updated; CI exercises both scripts.

Are there any user-facing changes?

The three binaries are no longer built by a default cargo build. Anyone who
ran them directly needs --features docs_generation — same as the dev/ scripts
now do. No library API changes.

If you would rather these lived outside the published crate altogether, moving
them to a small non-published dev/ crate would have the same effect on build
times; I went with the smaller change.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.17%. Comparing base (ab12f5e) to head (17ab914).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24338      +/-   ##
==========================================
+ Coverage   81.14%   81.17%   +0.03%     
==========================================
  Files        1112     1109       -3     
  Lines      386933   386746     -187     
  Branches   386933   386746     -187     
==========================================
- Hits       313967   313957      -10     
+ Misses      54476    54295     -181     
- Partials    18490    18494       +4     

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[comphead]

Thanks @Dandandan good catch