The README told readers to run `make nlohmann_json.docset`, but the
Makefile's targets are `all`, `JSON_for_Modern_C++.docset` and
`install_docset_zeal`; the documented command has failed with "No
rule to make target" since #2967 (2021). Point the README at the real
target and folder name. Info.plist's DashDocSetFallbackURL also still
pointed at the old nlohmann.github.io/json/ URL instead of the
canonical https://json.nlohmann.me/ from mkdocs.yml's site_url.
Leave list_missing_pages/list_removed_paths alone: they may become
redundant once #5638's check_docset() lands, which is a follow-up.
Part of #5718
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann