DOC: Update versioned Doxygen links to point to ReadTheDocs

These are now available for each release tag.

Author: thewtex

Documentation/docs/migration_guides/itk_5_migration_guide.md

@@ -69,8 +69,8 @@ Availability of the C++11 standard allows use of many Standard Library
 features. These were previously implemented as portable ITK classes.
 The standard library classes are preferred over ITK's implementations.
 The most notable examples of this are:
- * [atomic integers](https://itk.org/Doxygen413/html/classitk_1_1AtomicInt.html) should be replaced by `std::atomic`.
- * [mutex locks](https://itk.org/Doxygen413/html/classitk_1_1MutexLock.html)
+ * [atomic integers](https://docs.itk.org/projects/doxygen/en/v4.13.0/classitk_1_1AtomicInt.html) should be replaced by `std::atomic`.
+ * [mutex locks](https://docs.itk.org/projects/doxygen/en/v4.13.0/classitk_1_1MutexLock.html)
 and related classes should be replaced by the similarly named classes from STL's
 [`<mutex>`](https://en.cppreference.com/w/cpp/header/mutex) header.
  * `itksys::hash_map` should be replaced by [std::unordered_map](https://en.cppreference.com/w/cpp/container/unordered_map).

Documentation/docs/releases/2.0.md

@@ -16,7 +16,7 @@ formats are:
 -   ACR-NEMA version 1 and 2
 -   Dicom version 3.0 (including various encodings of jpeg, RLE)
 
-Class Reference : [GDCMImageIO](https://www.itk.org/Doxygen/html/classitk_1_1GDCMImageIO.html), [GDCMSeriesFileNames](https://www.itk.org/Doxygen/html/classitk_1_1GDCMSeriesFileNames.html)\
+Class Reference : [GDCMImageIO](https://docs.itk.org/projects/doxygen/en/v2.2.0/classitk_1_1GDCMImageIO.html), [GDCMSeriesFileNames](https://docs.itk.org/projects/doxygen/en/v2.2.0/classitk_1_1GDCMSeriesFileNames.html)\
 Examples : (None)
 
 ## New itkVesselTubeSpatialObject with metaIO file format to support vessel representation

Documentation/docs/releases/4.6.md

@@ -11,7 +11,7 @@ the Sphinx repository and the Wiki, which can now also be build as
 Remote modules. The Software Guide was split into two books and
 configure and build instructions were revised.
 
-\[1\] <https://www.itk.org/Doxygen/html/modules.html>
+\[1\] <https://docs.itk.org/projects/doxygen/en/v4.6.0/modules.html>
 
 New Features