docs: document HiDPI avatar srcset variants

[imorland]

Documents the HiDPI avatar srcset feature introduced in flarum/framework#4555.

avatars.md

• Updated DriverInterface section to document both avatarUrl() and the new avatarSrcset() method, with a full example showing density-descriptor srcset strings
• New section: HiDPI Variants for Uploaded Avatars — explains the @2x/@3x filename convention, the no-upscaling rule, and how the Avatar component handles srcset automatically
• New section: Providing HiDPI Avatars via OAuth Registration — documents provideAvatar2x() and provideAvatar3x() on the Registration object

update-2_0.md

• Added a Notable bullet summarising the HiDPI changes and linking to the avatar driver docs