docs(config-include): prepare for doc stabilization

[weihanglo]

What does this PR try to resolve?

Discussed in 2025-11-25 t-cargo meeting.
We prefer inline tables over array of tables for include.

• include is special and should appear early in config files,
  before other configuration keys
• Array-of-tables syntax allows interleaving with other config,
  which creates confusion about merge order and precedence
• Inline tables keep all include declarations together at the top,
  making the load order behavior clearer

Part of #7723 and #16284

How to test and review this PR?

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[weihanglo]

Failed on Windows x86_64 gnu. Retrying.

https://github.com/rust-lang/cargo/actions/runs/19680860094/job/56374258421?pr=16301#step:15:4756

 ---- freshness::move_target_directory_with_path_deps stdout ----
running `D:\a\cargo\cargo\target\debug\cargo.exe build`

thread 'freshness::move_target_directory_with_path_deps' (8876) panicked at tests\testsuite\freshness.rs:2399:54:
called `Result::unwrap()` on an `Err` value: Os { code: 5, kind: PermissionDenied, message: "Access is denied." }


failures:
    freshness::move_target_directory_with_path_deps

test result: FAILED. 3846 passed; 1 failed; 49 ignored; 0 measured; 0 filtered out; finished in 625.53s

error: test failed, to rerun pass `-p cargo --test testsuite`