Fix audit findings: bugs, security, perf, content

Bugs:
- setup-wsl: fix broken /img/WSL/* image paths (now bundle-relative;
  images were broken on the published post) + better alt text
- pi-docker-swarm: remove broken missing cluster.webp reference
- archetypes/posts.md: fix malformed {{ }} template so 'hugo new' works
- rename arrstack/arrstack.md -> index.md so it's a proper leaf bundle
- static/admin/config.yml: fix 'htpps' -> 'https' site_url typo
- fix silently-dropped <placeholder> text (unsafe=false dropped them):
  wrap <distro>/<username>/<user>/<version>/<ip address> etc in backticks
  across setup-wsl, securing-ssh, pi-docker-swarm, mc-server-on-docker

Security:
- render-link.html: add rel=noopener noreferrer to external target=_blank
- extend-footer.html: fix rel='me' trapped in class attr (broke Mastodon
  verification); add proper rel/target
- static/admin/index.html: pin Sveltia CMS to 0.172.4 via jsDelivr + SRI

Performance:
- hugo.toml: ignoreFiles for *.aseprite/*.bak (stop shipping source art;
  non-page files 23 -> 13)
- video shortcode: preload=metadata; image shortcode: loading=lazy decoding=async
- sws.toml: drop SPA page-fallback so missing paths return real 404 (404.html)

Content/SEO:
- normalize tag casing (HomeLab/RemoteAccess/SSH/HUGO -> lowercase),
  fix 'prodcutivity' typo, arrstack tags:[""] -> [], add welcome description
- delete stray index.md.bak

content/posts/securing-ssh/index.md

@@ -2,9 +2,9 @@
 title: Securing SSH 🤖
 description: Ever wanted to make your SSH connections more secure. Read this
 tags:
-  - HomeLab
-  - RemoteAccess
-  - SSH
+  - homelab
+  - remoteaccess
+  - ssh
 author: ahmza
 date: 2021-12-07
 draft: false
@@ -146,7 +146,7 @@ fine 😊
 - Max Auth Tries 5
 - PasswordAuthentication no
 - PermitEmptyPasswords no
-- AllowUsers <user>
+- AllowUsers `<user>`
 - ClientAliveInterval 180
 
 ``` bash