Document the images of json_documents

Add API pages for basic_json_document::save() and load(), including the
image_check enumeration and its three levels. Add examples that show
caching a parsed document as an image, loading it without parsing, the
difference between a borrowed and an owned image, and a full check
rejecting a damaged image that a bounds check still reads safely.

Add an "Images" section to the json_view feature page, register the new
pages in mkdocs.yml and docSet.sql, group basic_json_document's member
list by parsing/access/images/edits, and document parse_error.116 and
type_error.320 on the exceptions page, extending out_of_range.416 for
images.

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

docs/mkdocs/docs/home/exceptions.md

@@ -388,6 +388,23 @@ A UBJSON high-precision number could not be parsed.
     [json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A
     ```
 
+### json.exception.parse_error.116
+
+[`basic_json_document::load()`](../api/basic_json_document/load.md) rejected an
+[image](../features/json_view.md#images): either the bytes are not one [`save()`](../api/basic_json_document/save.md)
+could have written (too short, an unknown magic number or format version, or sizes that do not fit the buffer), or
+they are, but fail the requested [`image_check`](../api/basic_json_document/load.md#image_check).
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ## Iterator errors
 
 This exception is thrown if iterators passed to a library function do not match
@@ -800,6 +817,26 @@ from JSON text.
 
     This exception was added in version 3.13.0, together with editable [`json_document`s](../features/json_view.md).
 
+### json.exception.type_error.320
+
+[`basic_json_document::save()`](../api/basic_json_document/save.md) cannot write an
+[image](../features/json_view.md#images) of a [discarded](../api/basic_json_document/is_discarded.md) document.
+[`save()`](../api/basic_json_document/save.md) and [`load()`](../api/basic_json_document/load.md) also throw this
+exception on a big-endian target, since the image format is little-endian only.
+
+!!! failure "Example messages"
+
+    ```
+    [json.exception.type_error.320] cannot save a discarded json_document
+    ```
+    ```
+    [json.exception.type_error.320] json_document images need a little-endian target
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ## Out of range
 
 This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.
@@ -1027,7 +1064,9 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 so they do not support an input of 4 GiB or more. The same 32-bit limit applies to an **editable** document's own
 storage: [`set`](../api/basic_json_document/set.md) and [`push_back`](../api/basic_json_document/push_back.md) throw
 this exception once the strings and number tokens written by edits reach 4 GiB in total, or once more than
-4294967295 arrays/objects have had an element set or appended to them.
+4294967295 arrays/objects have had an element set or appended to them. The same limit applies to an
+[image](../features/json_view.md#images): [`save()`](../api/basic_json_document/save.md) throws it if the node
+count, the text, or the decoded strings it would write would individually reach 4 GiB.
 
 !!! failure "Example messages"
 
@@ -1037,6 +1076,9 @@ this exception once the strings and number tokens written by edits reach 4 GiB i
     ```
     [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document
     ```
+    ```
+    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document
+    ```
 
 !!! note