Point users to update(…, true), JSON Pointer defaults, and built-in std::optional (#5804)

A code search of client code found many hand-written versions of
functionality the library already has: recursive object merges
(update(j, true) since 3.10.5), dotted-path getters (value() and
contains() with a JSON Pointer), get_or helpers (value() throws for a
member that is present but null), and adl_serializer specializations
for std::optional (supported since 3.12.0).

- modifying_values.md: describe both modes of update(), add a
  defaults + user settings recipe, and stop recommending Merge Patch
  for recursive merges.
- merge_patch.md: note that null deletes keys, so a merge patch is not
  a general deep merge; link update().
- default_value.md: add "Nested values" (value/contains with a JSON
  Pointer, building a pointer from a dotted path with operator/=) and
  a warning that null and mistyped members are not missing.
- value.md: note that a null member is converted, not replaced by the
  default.
- json_pointer.md: link value/contains/at with JSON Pointers.
- arbitrary_types.md: note that std::optional needs no serializer.

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

docs/mkdocs/docs/features/json_pointer.md

@@ -77,6 +77,10 @@ auto val2 = j.at(json::json_pointer("/nested/three/1"));    // false
 auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
 ```
 
+To read a value with a fallback, use [`value`](../api/basic_json/value.md) with a JSON Pointer; to test for existence,
+use [`contains`](../api/basic_json/contains.md). Neither needs intermediate checks, see
+[Nested values](element_access/default_value.md#nested-values).
+
 !!! note "Creating intermediate levels that don't exist"
 
     See the [`operator[]` notes](../api/basic_json/operator%5B%5D.md#return-value) for how array vs. object is
@@ -126,6 +130,8 @@ auto j_original = j_flat.unflatten();
 ## See also
 
 - Class [`json_pointer`](../api/json_pointer/index.md)
+- Functions [`value`](../api/basic_json/value.md), [`contains`](../api/basic_json/contains.md), and
+  [`at`](../api/basic_json/at.md) accept JSON Pointers; see [Nested values](element_access/default_value.md#nested-values)
 - Function [`flatten`](../api/basic_json/flatten.md)
 - Function [`unflatten`](../api/basic_json/unflatten.md)
 - [JSON Patch](json_patch.md) - paths inside a patch are JSON Pointers