[Eventonica] Revise Part 3 to use Vanilla DOM JS instead of jQuery

[vegetabill]

File: https://github.com/Techtonica/curriculum/blob/main/projects/eventonica/eventonica-part3-jquery-ui.md

Changes

• Add prereq: https://github.com/Techtonica/curriculum/blob/main/web/js-dom-methods.md
• Change list of requirements into a link to the main README
• Remove "Pass an aXe accessibility test"
• Remove "Be deployed to GitHub pages"
• Remove Setup Step 1
• Find and replace the filename introduced in step 2 from jQuery-scripts.js to main.js
• Replace the example of a great README, https://gist.github.com/PurpleBooth/109311bb0361f32d87a2 (dead link) with a good one
• Move Next Steps "Deploying" and "Accessibility" to Challenges
• Update starter code index.htnml to use standard JS dom functions instead of jQuery
• Replace jQuery-specific code like $("#element-id").val() to use standard JS DOM functions