Address the review of the json_view object index and SIMD scan

From gregmarr's review of #5628:

- Docs and comments say that the hash index is one of the object's keys.
- object_index.hpp: conditional_static_cast for the hash, "slot" for the
  table position, no flag for a degenerate table, and a static_assert
  that the smallest indexed object is larger than the probe window.
- scan.hpp unrolls with NLOHMANN_VIEW_REPEAT_FIRST_8/SECOND_8.

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

docs/mkdocs/docs/api/basic_json_view/value.md

@@ -70,8 +70,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 1. Linear in the number of members: as for [`operator[]`](operator[].md#complexity), members are compared one after
    another, in document order, stopping at the first match. Plus the complexity of converting
    the found member to `T` (see [`get`](get.md)).
-   Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
-   on average.
+   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
+   constant time on average.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value