docs/hw-encoding: Add a note about the conda-distributed ffmpeg on windows

[peci1]

🦟 Bug fix

Summary

The ffmpeg distributed via conda only has these encoders on win64:

(D:\programming\gz9-ws\conda) D:\programming\gz9-ws\gz-ws>ffmpeg -hide_banner -encoders | grep 264
 V....D libx264              libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
 V....D libx264rgb           libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 RGB (codec h264)
 V....D libopenh264          OpenH264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
 V....D h264_mf              H264 via MediaFoundation (codec h264)
 V....D h264_nvenc           NVIDIA NVENC H.264 encoder (codec h264)

I've added an explicit notice that Windows users with Intel GPUs won't have HW acceleration available out of the box.

If somebody knows about an easy way to provide a custom ffmpeg build with h264_qsv, it would be good to add it to the general install instructions.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[azeey]

cc @traversaro