Remove unnecessary files and purge commit history

[tsmbland]

The repo currently contains a lot of files that either do not need to be committed to the repo (e.g. build files), or are due to be replaced (e.g. hands on exercises). We should delete these files and purge the commit history to shrink the repo.

Specific changes to make:

• _build/ and _deploy/ can be removed and added to .gitignore as these folders can be automatically generated
• case-studies/ can be removed (contains outdated model files)
• docs/hands_on_* can be removed (contains outdated exercises which will be replaced entirely by notebooks)
• review/ can be removed