Fix docset README's non-existent make target and stale fallback URL

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>

docs/docset/Info.plist

@@ -13,7 +13,7 @@
     <key>dashIndexFilePath</key>
     <string>index.html</string>
     <key>DashDocSetFallbackURL</key>
-    <string>https://nlohmann.github.io/json/</string>
+    <string>https://json.nlohmann.me/</string>
     <key>isJavaScriptEnabled</key>
     <true/>
 </dict>

docs/docset/README.md

@@ -7,10 +7,11 @@ documentation browsers like [Dash](https://kapeli.com/dash), [Velocity](https://
 The docset can be created with
 
 ```sh
-make nlohmann_json.docset
+make JSON_for_Modern_C++.docset
 ```
 
-The generated folder `nlohmann_json.docset` can then be opened in the documentation browser.
+The generated folder `JSON_for_Modern_C++.docset` can then be opened in the documentation browser. `make all` builds a
+`JSON_for_Modern_C++.tgz` archive instead, and `make install_docset_zeal` installs the docset for Zeal directly.
 
 A recent version is also part of the [Dash user contributions](https://github.com/Kapeli/Dash-User-Contributions/tree/master/docsets/JSON_for_Modern_C%2B%2B).