Point to new exercises

[jonathanpallant]

The middle section points to the old exercises. It should point to the new exercises instead.

As those exercises aren't published on-line, it will need to point to the markdown pages within the Github repo:

• https://github.com/ferrous-systems/rust-exercises/blob/main/exercise-book/src/simple-db.md
• https://github.com/ferrous-systems/rust-exercises/blob/main/exercise-book/src/connected-mailbox.md
• The TCP Echo Server is now supplied as a solution (but we can write it from scratch anyway)
• ⚠️ Multi-threaded mailbox is missing ⚠️