We have many chapters and subchapters.  Listing all of the subchapters
in the navigation sidebar by default can make it difficult to see the
high level structure of the Reference and can inhibit rapid navigation
based on that hierarchical structure.

Fortunately, mdbook has an option to fold the chapter listing.  This
causes only the top-level chapters to be shown by default while giving
the user the ability to unfold individual chapters.  Let's turn on
that option.

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Rollup merge of #146898 - rustbot:docs-update, r=ehuss

Update books

## rust-lang/book

1 commits in 3e9dc46aa563ca0c53ec826c41b05f10c5915925..33f1af40cc44dde7e3e892f7a508e6f427d2cbc6
2025-09-15 16:10:14 UTC to 2025-09-15 16:10:14 UTC

- Release trpl 0.3 (rust-lang/book#4505)

## rust-lang/reference

9 commits in b3ce60628c6f55ab8ff3dba9f3d20203df1c0dee..cc7247d8dfaef4c39000bb12c55c32ba5b5ba976
2025-09-20 10:26:26 UTC to 2025-09-08 18:07:29 UTC

- Document temporary scoping for destructuring assignments (rust-lang/reference#1992)
- Specify lifetime extension of `pin!` and `format_args!` arguments (rust-lang/reference#1980)
- update for more ABIs supporting c-variadics (rust-lang/reference#1936)
- Fix incorrect span tag (rust-lang/reference#1995)
- Remove strike attribute (rust-lang/reference#1997)
- Specify the target limits for target-specific ABIs (rust-lang/reference#2000)
- Remove tuple index carve out (rust-lang/reference#1966)
- Enable folding of chapter listing in navigation sidebar (rust-lang/reference#1988)
- Add support to grammar for single line comments (rust-lang/reference#1993)

## rust-lang/rust-by-example

1 commits in dd26bc8e726dc2e73534c8972d4dccd1bed7495f..2c9b490d70e535cf166bf17feba59e594579843f
2025-09-18 22:28:52 UTC to 2025-09-18 22:28:52 UTC

- Update unit testing output for additional test (rust-lang/rust-by-example#1958)

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