Update to Render Animation: Windows download of FFmpeg

Update the 'Windows' section of downloading FFmpeg to better explain process, add screenshots

Main changes are to improve clarity for users:

• Process of downloading zip file instead of .tar.gz file explained, with URL of download location added.
• File specifications of FFmpeg added with screenshot, to prevent confusion.
• Process of placing and unzipping file added.
• Explanation of providing path to Krita provided with additional screenshot.