feat: update vendor assets & refactor custom CSS path

- Upgrade MathJax from v2 to v3
- Updated Highlight, Leaflet, Fancybox, Mermaid, FontAwesome
- Fixed multilingual icon paths in Web App Manifest
- View Demo script now binds to all network I/F to enable testing on
  other physical devices like smart phones

BREAKING CHANGES:
Removed `plugins_css` from `params.toml`.
Override `custom.scss` to add custom styling.

Close #1400

scripts/view_demo.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env zsh
+
+HUGO_THEME=academic hugo --source exampleSite --themesDir ../../ --i18n-warnings --bind=0.0.0.0 -p 1315 --baseURL=http://0.0.0.0:1315 server