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.

[KevinRSX]

@ryanphuang I've applied your suggestions almost exactly as they are and added this semester's file. Thanks in advance for taking another look!

A non-goal for this PR: because we are not only presenting papers right now, we can probably update the description to include project progress and knowledge sharing. But then the table's format will have to change (currently it's tightly coupled with presenting published conference papers), so will the semester_page.html file that creates the table. I can create another PR for that. Doing it will take only a little time on my side, but perhaps you want this public website to focus more on the fact that we are discussing papers, so please let me know your preference.

[ryanphuang]

The current table format is fine. Project presentation and knowledge sharing just need the public title and leave columns like venue empty.