More documentation updates for 3.11.0 (#3553)

* mkdocs: add string_view examples

* mkdocs: reference underlying operators

* mkdocs: add operator<=> examples

* mkdocs: fix style check issues

* mkdocs: tweak BJData page

* mkdocs: add CMake option hints to macros

* mkdocs: fix JSON_DISABLE_ENUM_SERIALIZATION definition

* mkdocs: fix link to unit-udt.cpp

* mkdocs: fix "Arbitrary Type Conversions" title

* mkdocs: link to api/macros/*.md instead of features/macros.md

* mkdocs: document JSON_DisableEnumSerialization CMake option

* mkdocs: encode required C++ standard in example files

* docset: detect gsed/sed

* docset: update index

* docset: fix CSS patching

* docset: add list_missing_pages make target

* docset: add list_removed_paths make target

* docset: replace page titles with name from index

* docset: add install target for Zeal docset browser

* Use GCC_TOOL in ci_test_documentation target

docs/mkdocs/docs/api/basic_json/operator_le.md

@@ -15,7 +15,7 @@ bool operator<=(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
    according to the following rules:
     - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either
       operand is `NaN` and the other operand is either `NaN` or any other number.
-    - Otherwise, returns the result of `#!cpp !(rhs < lhs)`.
+    - Otherwise, returns the result of `#!cpp !(rhs < lhs)` (see [**operator<**](operator_lt.md)).
 
 1. Compares wether a JSON value is less than or equal to a scalar or a scalar is less than or equal
    to a JSON value by converting the scalar to a JSON value and comparing both JSON values according