[editorial] Add use of markdownlint-cli2 for linting, wrap some URLs in angle brackets

[chalin]

Note

Editorial chang only, consisting of adding <...> around certain URLs

• Formats markdown links containing calls to Hugo shortcodes using the <...> syntax so that the standard markdownlint tool can process such files
• Adds a script (check:markdown2) to invoke and dependencies for markdownlint-cli2. We'll ke this as a separate NPM script until I've ported the custom filter for use with markdownlint-cli2

Before running the formatted, there were no significant changes to the generated site files:

$ (cd public && g diff -bw -I 'prettier-ignore') | grep ^diff
diff --git a/site/index.html b/site/index.html

Important

From now on, if a URL contains a call to a Hugo shortcode, you must enclose the URL in angle brackets <...>.