Use `tree` as customization example

Laptop should be responsible for setting up the compilers, system package
manager, programming language version managers & installers, large system
dependencies such as Postgres, and any critical config for those tools which
is often overlooked but can cause bugs and headaches.

Other useful command-line tools such as `watch` and `tree` are better
installed in individuals' `~/.laptop.local` files.

Add `tree` as a good example for that file.

Try to make `~/.laptop.local` explanation clearer and more prominent by
placing it in a heading.

Remove Suspenders as an example. Right before using Suspenders, it's probably
best to re-install it to get the latest.