Restructure inline namespace and allow version component to be disabled (#3683)

docs/mkdocs/docs/features/namespace.md

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+# `nlohmann` Namespace
+
+The 3.11.0 release introduced an
+[inline namespace](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces) to allow different parts of
+a codebase to safely use different versions of the JSON library as long as they never exchange instances of library
+types.
+
+## Structure
+
+The complete default namespace name is derived as follows:
+
+- The root namespace is always `nlohmann`.
+- The inline namespace starts with `json_abi` and is followed by serveral optional ABI tags according to the value of
+  these ABI-affecting macros, in order:
+    - [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) defined non-zero appends `_diag`.
+    - [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md)
+      defined non-zero appends `_ldvcmp`.
+- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
+  underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
+  below.
+
+For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1` is:
+
+```cpp
+nlohmann::json_abi_diag_v3_11_2
+```
+
+## Purpose
+
+Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different
+definitions of [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This is illustrated in the diagram below.
+
+```plantuml
+[**nlohmann_json (v3.10.5)**\nJSON_DIAGNOSTICS=0] as [json]
+[**nlohmann_json (v3.10.5)**\nJSON_DIAGNOSTICS=1] as [json_diag]
+[**some_library**] as [library]
+[**application**] as [app]
+
+[library] ..|> [json]
+[app] ..|> [json_diag]
+[app] ..|>[library]
+```
+
+In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would
+result in a crashing application. If `some_library` never passes instances of JSON library types to the application,
+this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
+
+## Limitations
+
+Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked
+together and that include different versions and/or configurations of the JSON library – exchange and use library
+types.
+
+There is an exception when forward declarations are used (i.e., when including `json_fwd.hpp`) in which case the linker
+may complain about undefined references.
+
+## Disabling the version component
+
+Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and
+recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their
+own risk**, disable the version component of the linline namespace, allowing different versions – but not
+configurations – to be used in cases where the linker would otherwise output undefined reference errors.
+
+To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) to `1`.
+
+This applies to version 3.11.2 and above only, versions 3.11.0 and 3.11.1 can apply the technique described in the next
+section to emulate the effect of the `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.
+
+!!! danger "Use at your own risk"
+
+    Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect
+    behavior. You have been warned!
+## Disabling the inline namespace completely
+
+When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk**
+restore the old namespace layout by redefining
+[`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md) as
+follows:
+
+```cpp
+#define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {
+#define NLOHMANN_JSON_NAMESPACE_END    }
+```
+
+!!! danger "Use at your own risk"
+
+    Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You
+    have been warned!
+
+## Version history
+
+- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
+- Changed structure of inline namespace in version 3.11.2.