Address some issues from the accessibility milestone

[DragonSenseiGuy]

Change log:

• Added alt-text for the Welcome Video in the homepage
• Added alt-texts of "" for images that does not affect people with screen-reader in the events page as well as the timeline page
• Changes alt-texts in the events page form "AoC" to "Advent of Code" so, people who don't have knowledge about it can still know what it is.
• Change date in the 3.13 release from 2022 to 2024

Fixes

• Non-descriptive alt text on thumbnail on home page #1382
• No alt text for images on events page #1381
• Alt text on decorative images on timeline is useless #1406

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[coveralls]

coverage: 100.0%. remained the same
when pulling bc7b622 on DragonSenseiGuy:accessibility
into 023881d on python-discord:main.