Add "Previous sessions" section

[Sciencentistguy]

A json-related method was mentioned, and that would be pretty good.

[benslv]

Nearly done with this, although I've done that thing where you decide to completely reimplement the entire feature with a different method because it's probably better.

The plan will be to store each previous session as its own Markdown file in a directory. We can store frontmatter (metadata) at the top of each file (e.g. date, title, summary, image) as well as a body of text underneath that.

The Previous Sessions page can then render a bunch of cards based off the frontmatter from each of these files, and link to a dedicated page for each of the sessions, allowing people to add a little (or big!) write-up about the session, as well as linking to any helpful resources (videos, files etc.) that were used.

Much better and more extensible than the current method of using a chonky JSON file, I think.

Let me know what you think.

cc. @tomoneill19 @Sciencentistguy