[Refactor] Extract GetDisplayAvatarUrl in IUser

[zobweyt]

Description

This PR has a big effect on getting the user's avatar url and the need to create an extra extension or code duplication.

Here's how to get the display url of the user's avatar now, compared to the previous implementation:

- string url = user.GetAvatarUrl(format, size) ?? user.GetDefaultAvatarUrl();
+ string url = user.GetDisplayAvatarUrl(format, size);

Changes

• Extract GetDisplayAvatarUrl in IUser and complete it in heirs. (e51ef51)
• Fix webhook user's GetDisplayAvatarUrl implementation – now it will return the default avatar intead of null, which is quite obvious and corresponds to reality. (e51ef51)
• Remove obsolete GetAvatarAsync example in IUser.Examples. (d4b491c)
• Update avatar getters documentation for ease of reading. (6b5a7dc)

Related Issues

• Continues the idea of Add DisplayAvatarUrl() property to IGuildUser. #2113