Remove .html extension

[dichotommy]

Create VuePress permalink for each file so that there's no obnoxious .html extension. May also need to create redirects from the old .html links to the new permalink.

[guimachiavelli]

Given we will migrate to docusaurus, it's probably not worth the effort to do this now.

[dichotommy]

Because our migration to Docusaurus has been canceled in favor of sticking with VuePress for at least the next year, I'm reopening this.

[guimachiavelli]

Another change of plans 😅