Document that app.github.media is a real github domain

> [app.github.media](http://app.github.media) (no `s`) is apparently used by people at GitHub -- they decided it was a good idea to send email with links to it.

- https://github.com/github/docs/issues/42211#issuecomment-3757311851
> It was *really* weird. But as far as anyone can tell, it really does appear that GitHub was sending the email. If it is GitHub, it'd be really nice if GitHub made it clear. And if it isn't, then GitHub should do a trademark takedown of the domain (and then retain it). Either way, GitHub should clarify its control of the domain (and I believe that it already controls the domain).

- https://github.com/github/docs/issues/42211#issuecomment-3757322166
> It feels like they were using it for tracking link clicks, so if you have a marketing / announcements page that might be an ok place to hang the content.
>
> The related domain is:
https://images.github.media/EloquaImages/clients/GitHubInc/%7B97befb29-a5e2-404a-8a35-ddd059c235f2%7D_GitHub_Logo.png (from that email I forwarded):

https://docs.github.com/en/site-policy/github-terms/github-terms-of-service#a-definitions talks about some github domains...

Something in https://docs.github.com should mention that https://app.github.media/ is owned and operated by github.

- https://github.com/community/community/discussions/182061#discussion-9252686
- I've complained privately in the past.

- filed per https://github.com/github/docs/issues/42211#issuecomment-3806829039 and @subatoi in https://github.com/github/docs/issues/42211#issuecomment-5436332419

Start by reading the GitHub Terms of Service definitions section linked in the issue and examining the relevant docs.github.com site-policy pages. Add a clear statement that https://app.github.media/ is owned and operated by GitHub, with any related domain context needed. Done means the published documentation identifies the domain clearly and the links render correctly.

Written by the indexing model from the issue text.