* getting-started.md: Add title, link to /blog.
* guix-hpc.scm (image-url, css-url): Add leading /.
(post-url): Add "/blog".
(base-layout): Add "Blog", change feed URL.
(about-page): Remove.
(static-pages)[markdown-page]: New procedure.
Use it.  Add "getting-started".
* nginx-locations.conf: Add redirects.