Fix ~~ strikethrough by adding remark-gfm to markdown config

The ~~[Sold Out]~~ text showed as raw syntax because Astro's markdown processor doesn't include GFM by default. Added remark-gfm to remarkPlugins in astro.config.mjs, which enables strikethrough (plus tables, task lists, etc.). Reverted the sponsorship.mdx back to markdown ~~ syntax.

Author: nikoshell

astro.config.mjs

@@ -4,6 +4,7 @@ import { defineConfig } from "astro/config";
 import mdx from "@astrojs/mdx";
 import sitemap from "@astrojs/sitemap";
 import remarkToc from "remark-toc";
+import remarkGfm from "remark-gfm";
 import rehypeSlug from "rehype-slug";
 import rehypeAutolinkHeadings from "rehype-autolink-headings";
 import metaTags from "astro-meta-tags";
@@ -132,6 +133,7 @@ export default defineConfig({
   },
   markdown: {
     remarkPlugins: [
+      remarkGfm,
       [
         remarkToc,
         {

src/content/pages/sponsorship/sponsor.mdx

@@ -86,7 +86,7 @@ Alongside our main sponsorship packages, we offer **premium optional add-ons**
 designed to amplify your sponsorship impact and visibility.
 Available for **Silver Package level and above**, some popular examples include:
 
-- <del>**[Sold Out] Technical Talk Slot (30 min)**: €5000</del>
+- ~~**[Sold Out] Technical Talk Slot (30 min)**: €5000~~
 - **Tutorial/Workshop Slot (3h)**: €5000
 - **Poster slot**: €3000
 - **Open Space Sponsor**: €2000
@@ -100,7 +100,7 @@ Available for **Silver Package level and above**, some popular examples include:
 We also offer opportunities for
 <span class="font-semibold">Financial Aid Sponsor</span> –
 <span class="font-semibold">Social Event Sponsor</span> –
-<del><span class="font-semibold">[Sold Out] PyLadies Lunch Sponsor</span></del> –
+~~<span class="font-semibold">[Sold Out] PyLadies Lunch Sponsor</span>~~ –
 <span class="font-semibold">Speaker Dinner Sponsor</span> –
 <span class="font-semibold">Plenary Room Sponsor</span> –
 <span class="font-semibold">Quiet Room Sponsor</span> –