Document options to reduce compile times (#5611)

* Document options to reduce compile times

Add an integration page that collects the ways to reduce compile times
with measurements: json_fwd.hpp in headers, JSON_NO_AUTOMATIC_UDLS,
explicit instantiation with extern template, modules, and precompiled
headers, and notes that JSON_NO_IO and JSON_USE_GLOBAL_UDLS have no
measurable effect.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Include only json_literals.hpp in the JSON_NO_AUTOMATIC_UDLS examples

json_literals.hpp includes json.hpp itself, so including both is redundant.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/mkdocs.yml

@@ -108,6 +108,7 @@ nav:
       - integration/cmake.md
       - integration/package_managers.md
       - integration/pkg-config.md
+      - integration/compile_times.md
   - API Documentation:
       - basic_json:
           - 'Overview': api/basic_json/index.md