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/is_json_view.md

@@ -49,6 +49,7 @@ view class.
 - [basic_json_view](basic_json_view/index.md) - a handle to one value of a document
 - [get](basic_json_view/get.md) - which conversions read the view directly
 - [view_mode](view_mode.md) - read-only or editable
+- [Writing generic code](../features/json_view.md#writing-generic-code) - functions and `from_json()` for views
 
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