json_view: view_mode enum instead of bool Editable, node_kind for node::kind (#5919)

* json_view: view_mode enum instead of bool Editable, node_kind for node::kind

basic_json_document and basic_json_view take a view_mode (read_only, the
default, or editable) instead of a bool: basic_json_document<json, true>
did not say what true means. The shortcut aliases are unchanged.

The kind of a node is a node_kind (the value_t values plus link, 10 as
before), which removes the static_cast<std::uint8_t>(value_t::...) of
every comparison. The node and image layouts are unchanged.

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

* json_view: node_kind in the node writers, spell out read_only, comment fixes

The builder and the edit functions take and store a node_kind instead of
casting a value_t; the numbering assertions and the comments now speak of
node_kind, and the link kind notes that it follows value_t::discarded.
The read-only aliases name view_mode::read_only like the editable ones.
view_mode.md says the mode is a compile-time property of the type.

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

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

A read-only document has no edits, so a view of it reads correctly as a
view of an editable document before its first edit. The implicit
conversion lets a function taking a json_editable_view accept the views
of both modes. There is no conversion the other way: a read-only view of
an editable document would read wrong values after the next edit.

The mode comparisons in the cross-mode operator==/!= use an
is_other_mode trait, which astyle keeps compact.

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

---------

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

docs/mkdocs/docs/api/json_editable_view.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using json_editable_view = basic_json_view<json, true>;
+using json_editable_view = basic_json_view<json, view_mode::editable>;
 ```
 
 This type is a [`basic_json_view`](basic_json_view/index.md) of a value of a