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/operator[].md

@@ -83,8 +83,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
    another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
    key of a different length than `key` is rejected without touching the source text.
-   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 `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
@@ -116,7 +116,7 @@ document.
     member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
     `#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
     [`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
-    The hash index of a larger object (128 members or more) leads to the first member of a key as well.
+    The key hash index of a larger object (128 members or more) leads to the first member of a key as well.
     [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
     duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
     [`size()`](size.md#notes).