[Doc] Fix video playback on iOS

[slax57]

Problem

#8767 turned all GIF files into WebM files (to reduce assets size), but VP9 WebM is not yet supported on iOS Safari.

Solution

Leverage the <video> tag supporting multiple <source> with different types, and also encode all videos in mp4 (with H.265 HVEC codec, including the hvc1 tag for better compatibility with Apple software).

Misc

This PR also

• Adds back the GIF files to the repo, as it broke some other websites still referencing them
• Fixes the video maxWidth on mobile
• Sets the playsinline attribute on all videos to avoid opening the video fullscreen on mobile devices (which would not fit well with autoplay)
• Makes it easier to test on real mobile hadware with the --host 0.0.0.0 option to jekyll by default