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.