Adsk Contrib - Update ReadTheDocs build OS and DirectX header install

[doug-walker]

The CI Action was failing because RTD no longer supports ubuntu-20.04.

• Update build OS to ubuntu-lts-latest.
• Update requested Python from 3.11 to 3.14.

The Dependencies-latest Action was failing due to the windows-2025-vs2026 runner image apparently no longer having the DirectX headers available.

• Have the action install the headers via vcpkg.
• Adjust the based on the following suggestion from Claude:

When find_package(directx-headers CONFIG QUIET) finds the vcpkg-installed package, it creates the Microsoft::DirectX-Headers imported target with INTERFACE_INCLUDE_DIRECTORIES set. We now pull that path into DirectX-Headers_INCLUDE_DIR before find_package_handle_standard_args runs — so the required-variable check passes and DirectX-Headers_FOUND stays TRUE.

This also means the target-creation block at line 69 (if(DirectX-Headers_FOUND AND NOT TARGET Microsoft::DirectX-Headers AND DirectX-Headers_INCLUDE_DIR)) will correctly skip when the vcpkg config already created Microsoft::DirectX-Headers.

@num3ric, please review the DirectX fix, thank you!

[cozdas]

There is this warning in the readthedocs page about using the ubuntu-lts-latest tag. I think it's fine but still wanted to mention.

Warning
Using ubuntu-lts-latest may break your builds unexpectedly if your project isn’t compatible with the newest Ubuntu LTS version when it’s updated by Read the Docs.