[PATCH] meson: update doc logic for Windows

[Varghese, Vipin]

[AMD Official Use Only]

Hi Thomas,

<snipp>

Thank you for looking at this problem.

26/03/2022 03:59, Vipin Varghese:
> Support for shell scripts doxy-html-custom, generate_doxygen and 
> generate_examples are absent. The current patch address the same by 
> disabling document build notifying the user.

It should not prevent generating guides with sphinx.

We did get error from `doc/meson` stating ` echo command not available on windows` for both cmd and powershell for the line
```
run_target('doc', command: [echo, message, doc_target_names],
    depends: doc_targets)
```

> Steps to reproduce the error:
>  - Install dependencies doxygen & sphinix build on Windwos server 2019.
>  - Build DPDK with option enable_docs=true for API or User Guide.
>
> This produces error
> ```
> FAILED: doc/api/examples.dox
> sh -e dpdk/doc/api/generate_examples.sh dpdk/examples 
> doc/api/examples.dox ```

I suppose we could replace shell scripts with Python equivalent.

I am trying to minimize the changes as first step: fix the build error by disabling on windows`.
Next step we can convert to python to make it cross platform independent.