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/integration/index.md

@@ -43,6 +43,21 @@ using json = nlohmann::json;
 to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and
 Clang).
 
+!!! tip "Which file do I download?"
+
+    For a project without a package manager, the library itself, including the `_json` literals, is **one file**:
+    [`json.hpp`](https://github.com/nlohmann/json/releases/latest/download/json.hpp) from the
+    [releases page](https://github.com/nlohmann/json/releases). You do not need to clone the repository, build
+    anything, or link a library. Put the file in a directory named `nlohmann` that is on your include path (for
+    example `third_party/nlohmann/json.hpp` with `-I third_party`), so that `#include <nlohmann/json.hpp>` works.
+    Download the companion headers listed below only if you use them: `json_view.hpp` for the
+    [zero-copy views](../features/json_view.md), `json_fwd.hpp` for forward declarations, and `json_literals.hpp` if
+    you define `JSON_NO_AUTOMATIC_UDLS`. Keep them in the same `nlohmann` directory.
+
+    Do not copy only parts of the repository's `include/` directory: the headers in it include each other and must be
+    kept together. If your IDE reports "cannot open source file" while the compiler succeeds, the compiler's include
+    path is set correctly but the IDE's is not; add the same directory to the IDE's include paths.
+
 You can further use file
 [`single_include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_fwd.hpp)
 for forward declarations (see [Compile times](compile_times.md)), and file
@@ -55,6 +70,24 @@ types, additionally include
 [`single_include/nlohmann/json_view.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_view.hpp);
 see [Zero-copy JSON views](../features/json_view.md).
 
+## Notes for builds and distribution
+
+**Reproducible and path-independent builds.** The runtime assertions use `assert`, which embeds the name of the source
+file (`__FILE__`) in the binary. Release builds do not contain them, because they define `NDEBUG`. If you compile with
+assertions enabled and need binaries that do not depend on the build directory, either define `NDEBUG` or map the
+prefix of the path with the compiler option `-ffile-prefix-map=<old>=<new>` (GCC 8 and later, Clang 10 and later). See
+[Runtime assertions](../features/assertions.md).
+
+**There is no separate source file to compile.** The library is header-only, so there is no `.cpp` that could be
+built with different options than the code that includes it. It is also not available as a precompiled library. To
+compile the library with optimizations in an otherwise unoptimized build (for example to parse large files in a debug
+build, which is slow because of the unoptimized code and the checks of the debug standard library), keep the code that
+uses `json` in a separate source file and set the options for that file only. [Compile times](compile_times.md)
+shows how to [instantiate `basic_json` once](compile_times.md#instantiate-basic_json-once).
+
+**Debugger visualization.** The Natvis file works with the Visual Studio debugger, including the `cppvsdbg` engine in
+Visual Studio Code, but not with debuggers that wrap LLDB, such as `codelldb`; see [Debugging](../home/debugging.md).
+
 ## See also
 
 - [CMake](cmake.md) - integrate the library into a CMake project