Fix documentation gaps found in a full GitHub Discussions review

[nlohmann]

Summary

Reviewed all 1008 GitHub Discussions (2020–2026) to find recurring questions that better or more visible documentation would have avoided. This PR addresses ~26 distinct gaps identified in that review:

• New Debugging page collecting natvis, GDB pretty printer, LLDB status, and JSON_DIAGNOSTICS (previously scattered/undiscoverable)
• Thread-safety and schema-validation FAQ entries
• StringType's char-based requirement (no wstring/u16string/u32string)
• Brace-initialization-yields-arrays warning directly on the constructor reference page (previously only in the FAQ)
• std::any exclusion from get<T>(), with a manual-dispatch example
• Non-string-keyed std::map serializing as an array of pairs
• ordered_json compatibility with NLOHMANN_DEFINE_TYPE_* macros (already worked, was undocumented)
• std::array truncation on size-mismatched conversion (no exception)
• static_cast vs. get<std::optional<T>>() divergence
• Recipe for omitting a std::optional field instead of emitting null
• No built-in nesting-depth limit during parsing, plus a callback-based workaround recipe
• Recipe for streaming a large homogeneous array via parser callbacks
• operator>> stream-position semantics for concatenated JSON values
• JSON Pointer array-vs-object creation rule for non-existing paths
• CMake target name (nlohmann_json_modules) needed to link C++20 modules
• ESP-IDF/PlatformIO: no official package, link to a community fork
• get(key, default) as the Python dict.get() equivalent
• reserve() recipe for pre-allocating array capacity
• JSONC as an alias for the existing ignore_comments/ignore_trailing_commas combination (distinct from the unsupported JSON5)
• items() dereferenced-element type: decltype() idiom + detail-namespace stability caveat
• Various macro/type-conversion limitations (MSGPACK_DEFINE_ARRAY equivalent, char-array round-tripping, ADL serializer macro gap)

Each change traces back to a specific discussion thread; see the commit message for the full list.

Test plan

• Verified all new/modified relative markdown links resolve to existing files
• mkdocs.yml nav change reviewed for correct indentation (no local mkdocs build available to fully render the site)
• CI docs build / link-check (if any) — pending

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This PR was written by Claude Code as part of a broader GitHub Discussions review requested by @nlohmann.