Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/index.md

@@ -0,0 +1,56 @@
+# <small>nlohmann::</small>basic_json_document
+
+<small>Defined in header `<nlohmann/json_view.hpp>`</small>
+
+```cpp
+template<typename BasicJsonType>
+class basic_json_document;
+```
+
+A parsed JSON text, held as a flat index of its values
+([16 bytes per value](../../home/architecture.md#node-index-of-json-views)) instead of a tree of `BasicJsonType` values.
+Strings and numbers stay in the source text; only strings that contain escapes are decoded, into one buffer owned by
+the document. [`basic_json_view`](../basic_json_view/index.md) is a read-only handle to one value of a
+`basic_json_document`; [`materialize()`](../basic_json_view/materialize.md) turns a subtree back into the
+`BasicJsonType` value that [`BasicJsonType::parse()`](../basic_json/parse.md) would have produced for it.
+
+A document may **borrow** the text it was parsed from (the caller's buffer must then outlive the document) or **own**
+it (a copy, or an rvalue `#!cpp std::string` that was moved in); see [`owns_source`](owns_source.md). `basic_json_document`
+is move-only: copying a document would either duplicate a potentially large index and text, or leave two documents
+claiming to borrow the same buffer, so it is disabled.
+
+## Template parameters
+
+`BasicJsonType`
+:   a specialization of [`basic_json`](../basic_json/index.md), for instance [`json`](../json.md) or
+    [`ordered_json`](../ordered_json.md). Only 64-bit `number_integer_t`/`number_unsigned_t` types are supported; this
+    is checked with a `static_assert`.
+
+## Specializations
+
+- [**json_document**](../json_document.md) - documents of the default specialization [`json`](../json.md)
+- [**ordered_json_document**](../ordered_json_document.md) - documents of [`ordered_json`](../ordered_json.md)
+
+## Member types
+
+- **view_type** - the type of view returned by [`root()`](root.md) (`#!cpp basic_json_view<BasicJsonType>`)
+- **value_t** - the JSON type enumeration, see [`basic_json::value_t`](../basic_json/value_t.md)
+
+## Member functions
+
+- [(constructor)](basic_json_document.md)
+- [**parse**](parse.md) (_static_) - deserialize from a compatible input, borrowing or owning it as appropriate
+- [**parse_copy**](parse_copy.md) (_static_) - deserialize a copy of a compatible input
+- [**accept**](accept.md) (_static_) - check whether the input is valid JSON
+- [**read**](read.md) - (re-)parse into this document, reusing its memory
+- [**root**](root.md) - the view of the root value
+- [**is_discarded**](is_discarded.md) - return whether the last parse failed
+- [**source**](source.md) - the parsed text
+- [**owns_source**](owns_source.md) - return whether the document holds its own copy of the text
+- [**node_count**](node_count.md) - the number of index entries (values plus object keys)
+- [**memory_usage**](memory_usage.md) - the number of bytes held by the document
+- [**shrink_to_fit**](shrink_to_fit.md) - release unused index capacity
+
+## Version history
+
+- Added in version 3.13.0.