Set up workflow to build just the documentation.

[MarkCallow]

Make a separate pyktx.doc target for instead of bolting the commands to build the documents on to the pyktx target.

Fix CMake 3.31 warnings in the pyktx CMakeLists.txt.

Add lists of files to ignore for in each workflow file.

[MarkCallow]

@ShukantPal this PR adds a workflow to build just the KTX-Software documentation. For the Python binding I created a new pyktx.doc target separate from the pyktx target. But the custom command to build the documentation calls the pyktx buildscript.py which causes an attempt to link to libktx. I do not want to have to build libktx in this workflow. I tried commenting out the call to buildscripts.py but then I get errors like the following:

WARNING: autodoc: failed to import module 'ktx_hash_list' from module 'pyktx'; the following exception was raised:
No module named 'pyktx.native'

How can I build the documentation without building libktx?