doc: opt out of aggregated boostlook to solve toc issues

[Becheler]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

So the boostlook aggregated framework does not collapse table of contents. It makes navigation particularly hard.
Here we imitate exactly what Boost.Unordered is doing.
We have full control over https://github.com/boostorg/graph-ui-bundle
It means we can control the ToC rendering and reinstate the warning banner saying the documentation is being rewritten.

Motivation

Testing

By leaving boostlook I believe we lose access to PR previews (they were not truthfull to final rendering anyway)
So just merge and see how it renders on develop and/or master 😢

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[cppalliance-bot]

An automated preview of the documentation is available at https://521.graph.prtest3.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-07-09 09:00:33 UTC

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 29006529514 vs develop run 28634825303 (8286c075b3).

Job	Baseline	After	Delta
macos (clang, 14)	4	44	+40
macos (clang, 17)	4	4	0
macos (clang, 20)	4	44	+40
ubuntu (clang-19, 14)	4	4	0
ubuntu (clang-19, 17)	4	4	0
ubuntu (clang-19, 20)	4	4	0
ubuntu (clang-19, 23)	4	4	0
ubuntu (gcc-14, 14)	10	10	0
ubuntu (gcc-14, 17)	10	10	0
ubuntu (gcc-14, 20)	10	10	0
ubuntu (gcc-14, 23)	10	10	0
windows_msvc_14_3 (msvc-14.3)	971	971	0