fix: restructure visitors pages to match old html style

[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

Revisit modern documentation of visitors to match the old style html.

Motivation

There were some deviations from old style during migration (e.g. tables removed) that made it harder to navigate. This PR intends to remediate it.

Testing

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://517.graph.prtest3.cppalliance.org/index.html

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

2026-06-24 12:10:13 UTC

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 28097204036 vs develop run 28012997199 (3898362db3).

Job	Baseline	After	Delta
macos (clang, 14)	44	4	-40
macos (clang, 17)	4	44	+40
macos (clang, 20)	4	4	0
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