Improve index for the C API

[serhiy-storchaka]

Index entries are added either implicitly by directives like c:function or explicitly by the index directive. And some explicitly added index entries do not match implicitly added index entries. For example:

   single: PyList_GetItem()

creates index entry PyList_GetItem(), but the function declaration creates index entry PyList_GetItem (C function). In result, two entries are not merged.

The following PR makes explicit index entries look like automatically generated ones.

Linked PRs

• gh-115172: Fix explicit index extries for the C API #115173
• [3.12] gh-115172: Fix explicit index extries for the C API (GH-115173) #115292
• [3.11] gh-115172: Fix explicit index extries for the C API (GH-115173) #115293