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/features/parsing/parse_exceptions.md

@@ -21,6 +21,10 @@ catch (json::parse_error& ex)
 In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
 
 
+!!! tip
+
+    [Working without exceptions](../no_exceptions.md) also covers accessing values and converting to your own types.
+
 ## Switch off exceptions
 
 The [`parse()`](../../api/basic_json/parse.md) function accepts a `#!cpp bool` parameter `allow_exceptions` which
@@ -81,3 +85,61 @@ The return value indicates whether the parsing should continue, so the function
     ```
     --8<-- "examples/sax_no_exception.output"
     ```
+
+## Where did the error occur?
+
+The message of a [`parse_error`](../../home/exceptions.md#jsonexceptionparse_error101) for JSON text contains the line
+and column of the error:
+
+```
+[json.exception.parse_error.101] parse error at line 3, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
+```
+
+The exception object itself only stores the **byte index** of the last read character in the member `byte` (the first
+byte of the input has index 1). The same index is passed to `parse_error` in the [SAX interface](sax_interface.md). If
+you need the line and column as numbers, for example to show them in an editor, compute them from the input:
+
+??? example "Example: compute line and column of a parse error"
+
+    ```cpp
+    --8<-- "examples/parse_error_position.cpp"
+    ```
+
+    Output:
+
+    ```
+    --8<-- "examples/parse_error_position.output"
+    ```
+
+Columns count bytes, so a line with multi-byte UTF-8 characters before the error may differ from the column your editor
+shows.
+
+### Where is the value that caused a later error?
+
+A `parse_error` is raised while the text is read. Many mistakes show up later: the text is valid JSON, but a value has
+the wrong type when you convert it (`type_error.302`), or a key is missing (`out_of_range.403`). The exception then says
+what is wrong, but not where in the input the value was. Define
+[`JSON_DIAGNOSTIC_POSITIONS`](../../api/macros/json_diagnostic_positions.md) before including the library to add the
+byte range of the offending value to such messages (since version 3.12.0), and to query the range of any value with
+[`start_pos()`](../../api/basic_json/start_pos.md) and [`end_pos()`](../../api/basic_json/end_pos.md):
+
+??? example "Example: locate a value with diagnostic positions"
+
+    ```cpp
+    --8<-- "examples/parse_error_position_diagnostic.cpp"
+    ```
+
+    Output:
+
+    ```
+    --8<-- "examples/parse_error_position_diagnostic.output"
+    ```
+
+Positions are only available for values created by [`parse`](../../api/basic_json/parse.md), they are not updated when a
+value is changed, and the macro makes every value 16 bytes larger on 64-bit platforms, so use it where the cost is
+acceptable, for example for configuration files. Note that `start_pos()` is a 0-based index into the input, whereas
+`byte` of a `parse_error` starts at 1.
+
+For binary formats such as CBOR, there are no lines; `byte` is the position in the byte sequence. To find out how many
+bytes a successfully parsed value used, see
+[Several values in one stream](large_and_partial_input.md#several-values-in-one-stream).