Change website theme to minima (#1316)

Another attempt at switching to a better theme (with dark mode) for the OSSU CS github pages website. It looks like this: https://pulkitkrishna.eu.org/computer-science/

Changes:
- Change the theme to `minima`, the default jekyll theme (jekyll is the static site generator used by github pages). This supports a dark theme. if your OS and Browser is in dark mode, the website will appear with a dark theme.
- Made some changes to the theme's components to better fit the OSSU github repository layout, and not require any changes to the markdown files themselves. The goal is to not affect anyone who uses the github repository instead, in any way.
- Added a favicon
- Change the OSSU banner and the "keep learning" image to local webp images, instead of png/jpeg hosted on imgur, to make them load faster.
- Remove the mention of curriculum version number.

_includes/nav-items.html

@@ -0,0 +1,7 @@
+{%- for path in include.paths -%}
+  {%- assign hyperpage = site.pages | where: "path", path | first -%}
+  {%- if hyperpage.title %}
+  <a class="nav-item" href="{{ hyperpage.url | relative_url }}">{{ hyperpage.title | escape }}</a>
+  {%- endif -%}
+{%- endfor -%}
+<a class="nav-item" href="https://github.com/ossu/computer-science">GitHub</a>