fix(layout): make content width respect page.width config (#911)

Fixes #886 - The content area in `<main>` was hardcoded to `hx:max-w-6xl`
(72rem), ignoring the `params.page.width` setting. Added a new CSS
variable `--hextra-max-content-width` that responds to the page width
config, so users can now get full-width or wide content as expected.

docs/hugo_stats.json

@@ -129,6 +129,7 @@
       "hextra-jupyter-code-cell-outputs-container",
       "hextra-language-options",
       "hextra-language-switcher",
+      "hextra-max-content-width",
       "hextra-max-footer-width",
       "hextra-max-navbar-width",
       "hextra-max-page-width",
@@ -476,7 +477,6 @@
       "hx:max-md:min-h-[340px]",
       "hx:max-md:sticky",
       "hx:max-sm:grid-cols-1",
-      "hx:max-w-6xl",
       "hx:max-w-[50%]",
       "hx:max-w-[90rem]",
       "hx:max-w-[min(calc(100vw-2rem),calc(100%+20rem))]",