Docs: fill documentation gaps found in user questions (#5907)

* Docs: fill documentation gaps found in user questions

Add pages on class-hierarchy/smart-pointer/variant/chrono recipes, working
without exceptions, and large, streamed and partial input. Extend the FAQ
(23 entries, Known bugs first), parse error documentation (empty-input
checklist, line/column, diagnostic positions), element access, conversions,
creating values, ADL troubleshooting, memory, and build/distribution notes.
Add 14 examples with generated output. Correct the statement that parse errors
cannot be recovered from with -fno-exceptions.

The topics come from recurring questions on Stack Overflow, Reddit, Hacker
News, YouTube, and the project's GitHub discussions and issues.

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

* docs: declare range-for lists in variables to keep astyle quiet

astyle reformats a braced init list placed directly in a range-for
header; declare the list first so check_amalgamation passes.

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

---------

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

docs/mkdocs/docs/index.md

@@ -31,6 +31,10 @@ int main()
     j["name"] = "Niels";
     j["list"] = {1, 0, 2};
 
+    // read a value into a C++ variable and use it
+    double pi = j["pi"].get<double>();
+    std::cout << "2 * pi = " << 2 * pi << '\n';
+
     // serialize with an indent of 4 spaces
     std::cout << j.dump(4) << '\n';
 }