Add slides for the course

[jatkinson1000]

Slides currently render at https://cambridge-iccs.github.io/intermediate-git
We may want to remove the action from deploying from this PR once ready to merge.

[jatkinson1000]

@Mikolaj-A-Kowalski I think these are now ready for review with feedback incorporated.

A couple of options when merging:

• squash into a single commit at the end - allows easy tweaking with --amend in future
  • But users will see this in the git log during rebase/bisect
• rebase to be before the code
  • hidden further from users, but harder to amend in future (fixups and force pushes to main)
• Leave on a separate branch
  • Slides not entangled with exercise commits, but leaves multiple branches to be managing.