Consistent way to refer to ITK classes, namespaces, and ITK in Doxygen

### Description

[Doxygen automatically links](https://doxygen.nl/manual/autolink.html) to [documented classes](https://doxygen.nl/manual/autolink.html#linkclass):
```
All words in the documentation that correspond to a documented class and contain at least one non-lower case character
will automatically be replaced by a link to the page containing the documentation of the class.
```

Some classes mention other ITK classes as fully qualified names, e.g. `itk::Image`, e.g.
https://itk.org/Doxygen/html/classitk_1_1ImageToVTKImageFilter.html

whereas other simply use `Image`, e.g.
https://itk.org/Doxygen/html/classitk_1_1CovariantVector.html

Additionally, decide whether the case used for ITK will be lowercase (`itk`) or uppercase (`ITK`). Note that neither gets linked to the [itk namespace](https://itk.org/Doxygen/html/namespaceitk.html) by Doxygen. Maybe there is a way to do it: maybe using the [`\ref` command](https://doxygen.nl/manual/autolink.html#linkclass):
"To link to an all lower case symbol, use [\ref](https://doxygen.nl/manual/commands.html#cmdref)."

It should be decided in which cases we will be willing to link to the ITK namespace. The can apply to [other namespaces](https://itk.org/Doxygen/html/namespaces.html).

**The decisions should be documented in the ITK Software Guide.**

### Expected information

Consistent way to refer to ITK classes, namespaces, and ITK itself.

### Actual information

Inconsistent way to refer to ITK classes,, namespaces, and ITK itself.

### Versions

`master`

### Additional Information

None.