docs: port to VitePress

Things that happen in this shameless squash:
- Removed 404 page
  - It doesn't work in VitePress and making it work in VitePress needs
    some invasive hack in the theme that I'm not willing to do
  - The default is more than enough, though
- Moved consoleVersionSelect include to another directory
  - This avoids being uploaded to Crowdin for translation
- Inline all SafeB9SInstaller includes
  - Not only do parameterized includes not work, but if/else statements
    don't work at all
- The sysupdate include parameter hasn't been used since MSET9, so it
  actually works just fine without any parameters. Hooray(?)
- Move mset9-chorus to an include, that logic doesn't work in VitePress
- Removed Get Started button from homepage, it breaks translations
- Tables in get-started are now shown by default; it seems VitePress really does
  not want us to use inline script. So we cannot enable it upon
  realizing that the browser is JS capable
- Include parameters are now in frontmatter to allow translation
- Hacked in a bunch of CSS values kanged from the old theme to make it
  look not stupid
- Drop headers down one level to make table of contents work

docs/installing-boot9strap-(mset9).md

@@ -1,10 +1,6 @@
----
-title: "Installing boot9strap (MSET9)"
----
+# Installing boot9strap (MSET9)
 
-{% include toc title="Table of Contents" %}
-
-### Choose your operating system
+## Choose your operating system
 
 The instructions for MSET9 differ depending on the computer, phone, or tablet **that you are using to mod your console**.