docs: Add warning regarding YouTube account risks when using cookies

[HenryLai921]

Added a warning note regarding the risks of account flagging when passing cookies to YouTube, based on Known Issues #3766.
Relates to #15106

[bashonly]

Note that we do already have a big CAUTION sign at the top of the "exporting youtube cookies" section:

https://github.com/yt-dlp/yt-dlp/wiki/Extractors#exporting-youtube-cookies

[ruben1PvP]

The #67 PR will also add a warning in the "How do I use my account" section:

**CAUTION:** You shouldn't always use your account's cookies, but only in specific scenarios:

1. The video is age-restricted (and you can watch age-restricted videos with your account). 
1. The video is paywalled/members-only (and you have a membership in that channel with your account).
1. Your account has a premium subscription (e.g. YouTube Premium or Soundcloud Go(+)) and you want to download premium formats.
1. You have been IP restricted and you want to continue downloading without changing your IP (explained [here](https://github.com/yt-dlp/yt-dlp/wiki/Extractors#common-youtube-errors) for YouTube).

In cases where you need to use your account's cookies, avoid using an important account's cookies, as this may result in the account being restricted. Sites like Instagram have ways to detect automatic behaviour in your account (like downloading with yt-dlp) and can restrict your account after just a few downloads.