Fixes#788#790#1074#1252#1364

• Fix inability to instantiate reactive Vue components by 1) handling each child of#main instead of#main itself and 2) skipping elements that are already Vue instances
• Retain previous behavior of handling basic Vue rendering without the need for a markdown<script> tag.
• Add Vue3 support
• Update documentation and add live Vue examples
• Updateindex.html files to include Vue.js
• UpdateCypress Jest+Playwright e2e tests

Summary: Docsify was mounting#main as a Vue component on every page change when it detected Vue.js, presumably to process basic Vue content without requiring a<script> tag in each markdown file. The issue is that this was happeningafter users had already instantiated child elements of#main as Vue components, destroying those instances.