Add Kobweb support for creating a sitemap

[bitspittle]

Note we already have this comment detailing how one might do it:

kobweb/tools/gradle-plugins/application/src/main/kotlin/com/varabyte/kobweb/gradle/application/util/site/ProjectExtensions.kt

Line 17 in a694950

* tasks.register("createSitemap") {

What I don't know is if sitemaps are pretty standard or generally very custom per site. If pretty standard, we should add something like kobweb { app { generateSitemap() } }. If custom, then we should add an article to the docs page.

[kocheick]

I'm about to tackle this issue next.