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/untrusted_input.md

@@ -26,9 +26,11 @@ several alternatives; see [Parsing and exceptions](parse_exceptions.md) for the
   [example](parse_exceptions.md#user-defined-sax-interface) that overrides it to print instead of throw.
 
 If exceptions are unavailable entirely (`-fno-exceptions`, or [`JSON_NOEXCEPTION`](../../api/macros/json_noexception.md)
-defined), every `#!cpp throw` in the library becomes a call to `std::abort()` -- there is no way to recover from a
-parse error of untrusted input in that configuration; see
-[Switch off exceptions](../../home/exceptions.md#switch-off-exceptions) for the details and for overriding this with
+defined), every `#!cpp throw` in the library becomes a call to `std::abort()`. The alternatives above do not throw, so
+`parse()` with `allow_exceptions` set to `#!cpp false` (and likewise `accept()` and the `from_*` functions with
+`allow_exceptions` set to `#!cpp false`) can still reject invalid input in that configuration; a plain `parse()` call
+aborts on invalid input. See [Working without exceptions](../no_exceptions.md) and
+[Switch off exceptions](../../home/exceptions.md#switch-off-exceptions) for the details and for overriding the abort with
 `JSON_THROW_USER`.
 
 ## Nesting depth