docs: update Doxygen documentation URLs to doxygen.nl domain

These changes were introduced in bitcoin#15514 (Update Transifex links),
which ordinarily do not apply to Dash as it uses its own Transifex
account but not mentioned in the name are updates to Doxygen URLs.

Author: kwvg

doc/developer-notes.md

@@ -189,7 +189,7 @@ For example, to describe a function use:
 bool function(int arg1, const char *arg2, std::string& arg3)
 ```
 
-A complete list of `@xxx` commands can be found at http://www.stack.nl/~dimitri/doxygen/manual/commands.html.
+A complete list of `@xxx` commands can be found at http://www.doxygen.nl/manual/commands.html.
 As Doxygen recognizes the comments by the delimiters (`/**` and `*/` in this case), you don't
 *need* to provide any commands for a comment to be valid; just a description text is fine.
 
@@ -229,7 +229,7 @@ Not picked up by Doxygen:
 //
 ```
 
-A full list of comment syntaxes picked up by Doxygen can be found at https://www.stack.nl/~dimitri/doxygen/manual/docblocks.html,
+A full list of comment syntaxes picked up by Doxygen can be found at https://www.doxygen.nl/manual/docblocks.html,
 but the above styles are favored.
 
 Recommendations: