docs: fixup absolute link warning for {{since(nightly)}}

We can now compute a relative link instead

mkdocs_macros.py

@@ -10,12 +10,15 @@ def define_env(env):
     @env.macro
     def since(vers, outline=False, inline=False):
         if vers == "nightly":
+            # Determine the relative path traversal to the root,
+            # so that we can emit the link to the install page
+            rel_root = "../" * (len(env.page.url.split('/')) - 1)
             first_line = "*Since: Nightly Builds Only*"
             expanded = "+"
-            blurb = """
+            blurb = f"""
     The functionality described in this section requires a nightly build of wezterm.
     You can obtain a nightly build by following the instructions from the
-    [Download](/wezterm/installation.html) section.
+    [Download]({rel_root}installation.md) section.
 """
         else:
             first_line = f"*Since: Version {vers}*"