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/examples/operator_spaceship__const_reference.c++20.output

@@ -0,0 +1,4 @@
+[1,2,3] <=> [1,2,4] := less
+{"A":"a","B":"b"} <=> {"A":"a","B":"b"} := equivalent
+"foo" <=> 17 := greater
+"foo" <=> <discarded> := unordered