Document options to reduce compile times

[nlohmann]

Stacked on #5610. Adds the documentation page Integration › Compile times, which collects the options discussed in #5294 to reduce compile times, each with measurements (C++17, Apple clang and GCC 16, -O0/-O2, median of 9 runs):

Option	Where it helps	Measured change
json_fwd.hpp in headers	headers that only name json	−53 % (clang), −69 % (GCC)
JSON_NO_AUTOMATIC_UDLS (#5610) + json_literals.hpp where needed	TUs that don't parse	−14 … −19 % (up to about −33 % for include-only TUs); parsing TUs unchanged
extern template class nlohmann::basic_json<>; + one explicit instantiation TU	TUs using json	−4 … −10 % (clang), −20 … −25 % (GCC); the instantiation TU takes 2–5 s (clang) / 5–11 s (GCC), once
C++20 modules	everywhere	refers to the existing Modules page (experimental)
Precompiled headers	everywhere	general CMake hint, not measured
JSON_NO_IO, JSON_USE_GLOBAL_UDLS=0	—	no measurable effect (listed explicitly so people don't reach for them)

I compiled and linked the extern template example with clang (C++11/17) and GCC 16 (C++17). The page also notes that member templates (get<T>(), parse(...)) are not covered by the explicit instantiation, and that ordered_json needs its own lines.

Also links the page from the integration index and the JSON_NO_AUTOMATIC_UDLS page, and adds it to the navigation.

Public API

No changes. This is documentation only.

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This PR description was written by Claude Code.

🤖 Generated with Claude Code

[gregmarr]

json_literals.hpp includes json.hpp so this is promoting extra cost. Either json_literals.hpp doesn't include json.hpp or this recommends just including json_literals.hpp. I would be fine with either version.

[nlohmann]

Thanks, done in 2ce78e1: the example (here and in the JSON_NO_AUTOMATIC_UDLS page) now includes only <nlohmann/json_literals.hpp> and mentions that it brings in <nlohmann/json.hpp>.

(Written by Claude Code.)

[gregmarr]

One minor suggestion.