docs: writing generic code for json_view (#5924)

* docs: writing generic code for json_view

A "Writing generic code" section on the json_view page: why a function
taking json_view accepts no other view, the three signatures for
editable, read-only, or either kind of view (passed by value), why the
modes do not convert, is_json_view, and how a from_json() reads views
(constrained to json values and views; the macros and the library's
conversions do it already) with what differs then (duplicate keys,
JSON_DIAGNOSTICS paths).

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

* docs: name the documents the generic view function accepts

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

* docs: views of read-only documents convert to editable views

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

---------

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

docs/mkdocs/docs/api/basic_json_view/get.md

@@ -38,7 +38,7 @@ as [`BasicJsonType::parse()`](../basic_json/parse.md) does), as well as the type
 accept views, e.g. with [`is_json_view`](../is_json_view.md). A `from_json()` that also accepts types that are no JSON
 values at all, such as the unconstrained `#!cpp template<typename BasicJsonType> void from_json(const BasicJsonType&,
 T&)` of [arbitrary type conversions](../../features/arbitrary_types.md), is not called with a view: it may use
-`BasicJsonType` functions a view does not have. See [json_view](../../features/json_view.md).
+`BasicJsonType` functions a view does not have. See [Writing generic code](../../features/json_view.md#writing-generic-code).
 
 !!! warning "Duplicate keys"