Post-migration website tweaks

[justinvp]

Some follow-ups:

• (justinvp) Hook up Purgecss to remove unused CSS classes (Use Purgecss to reduce the size of our CSS #1291)
• Fix styles used on AWS Crosswalk images to use Tailwind
• Looks like we have both /image/pulumi.png and /social/pulumi.png used as meta images. We should standardize on /image/pulumi.png and update any redirects as appropriate. (Replace RELATIVE_TO_PAGE with Page Resource helpers #1374)
• Implement a two-column layout for the docs landing page. Consider implications on search.
• Follow-up improvements to "On this Page": generate for "Download and Install", add sticky positioning