fix(web/blog): make base URL configurable for atom feeds

This was previously hardcoded to tazj.in, which is not going to work
of course.

Instead it now takes the blog config which has a new baseUrl
parameter. For ease of use, the configs of my and the TVL blog have
been moved into a location that is accessible in the tree for reuse.

Change-Id: I94e71aaa7859db4380eb7013740a17f6b6a02620
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3777
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/tvl/blog/default.nix

@@ -1,6 +1,12 @@
 { depot, ... }:
 
 {
+  config = {
+    name = "TVL's blog";
+    footer = depot.web.tvl.footer {};
+    baseUrl = "https://tvl.fyi/blog";
+  };
+
   posts = [
     {
       key = "kicking-off-tvix";