upgrade: Use Jekyll; style and content updates

[KevinRSX]

This PR does not change the website, including the content and style, though I plan to add another PR to improve those as well. What it does:

• Use Jekyll, allowing a easy process to add new semesters and new meetings in a semester. The coordinator only needs to edit the header of a Markdown file, instead of handling the HTML texts, which is error-prone
• Change the base URL to /reading-group/ and have it redirect to the current semester, which can be set in the config file, instead of /reading-group/current/
• To change the description of a semester, e.g., the link of the signup sheet, adding the requirements of a new topic, etc., can simply edit the same Markdown file
• The old HTMLs are currently placed in the archive/ folder

@ryanphuang I cannot test how the website looks like because GitHub Page's CI tracks only the main branch for deployment. Can you help take a look at whether these changes are plausible? Thanks!

There are too many file changes, please don't request Copilot to review this PR!

[ryanphuang]

Two major changes: (1) don't use redirect; let index.html directly render the latest semester page; (2) make semesters file into collections.

[ryanphuang]

Make semesters into collections to avoid this manual specification

1. mv semesters _semesters
2. Replace the semester config block in _config.yml with

collections:
  semesters:
    output: true
    permalink: /reading-group/:name/

[ryanphuang]

Put this file to _includes, so it can be used to render the root index.html as well. Several changes are needed.

1. Move the file to _includes/semester_page.html.
2. Replace all the page.* with something like include.sem.*, e.g.,

-  <meta name="description" content="OrderLab Reading Group, {{ page.semester }}">
+  <meta name="description" content="OrderLab Reading Group, {{ include.sem.semester }}">
   <link rel="stylesheet" href="{{ site.baseurl }}/assets/css/site.css">
   <link rel="icon" type="image/vnd.microsoft.icon" href="{{ site.baseurl }}/assets/favicon.ico">
-  <title>{{ page.semester }}</title>
+  <title>{{ include.sem.semester }}</title>

3. Update the way to loop the semesters:

-      {% for sem in site.semesters %}
-        <li{% if sem.id == page.semester_id %} class="current-semester"{% endif %}>
-          <a href="{{ site.baseurl }}/{{ sem.id }}/">{{ sem.name }}</a>
-        </li>
-      {% endfor %}
+        {% assign all_sems = site.semesters | sort: "date" | reverse %}
+        {% for sem in all_sems %}
+          <li{% if sem.url == include.sem.url %} class="current-semester"{% endif %}>
+            <a href="{{ sem.url | relative_url }}">{{ sem.semester }}</a>
+          </li>
+        {% endfor %}

This will use the tag inside the semester page, instead of the previous manual metadata in the _config.yml. It also automatically sort the semester pages by date, so make sure you add a date field in side each file.

4. Create the _layouts/semester.html to be simply:

{% include semester_page.html sem=page %}

5. Create another layout for the root index.html, e.g., _layouts/home.html

{% assign latest = site.semesters | sort: "date" | last %}
{% include semester_page.html sem=latest %}

This will automatically use the latest semester page.

[ryanphuang]

Add a date field (like 2025-08-26) for each file to support sorting and finding latest page.

[ryanphuang]

Don't use redirect for this use case. It's not a good practice. With the suggested new layout page, e.g., _layout/home.html, you just need the following for the root index.html:

---
layout: home
---