Implement Seed-level video capture setting handling + Job-level PDF-only option

[gretchenleighmiller]

This PR covers the following:

• Adds a new video_capture configuration option on the Seed level. This has four possible values; see newly added documentation for details.
• Implements the video_capture option, which impacts yt-dlp extraction and MIME types of outlinks. The remainder of video capture handling is accomplished in warcprox via Warcprox-Meta headers.
• Removes the previous file-based skip_av_seeds functionality.
• Adds a new pdfs_only configuration option on the Job level. This is a boolean that defaults to False; see newly added documentation for details.
• Implements the pdfs_only option based on the MIME type of outlinks. The remaining PDF-only filtering is accomplished in warcprox via Warcprox-Meta headers.
• Documentation updates and minor style cleanup.

[galgeek]

@gretchenleighmiller, thanks for your work on this!

I've left a couple of comments it might be good to address.