Document steps necessary to complete functional setup when using option `--skip-setup`

[jsejcksn]

This addresses the persona of a user who wants to try a new tool, but doesn't want to have their filesystem mutated in unexpected ways.

At Advanced > Installers > Skipping Volta Setup, it currently shows:

Skipping Volta Setup

If you wish to run the installer but do not want your profile scripts modified by volta setup, you can pass the --skip-setup option to the installer:

curl https://get.volta.sh | bash -s -- --skip-setup

NOTE
We don't currently support skipping volta setup on Windows.

However, this leaves the user's installation in a non-functional state and doesn't provide any guidance on how to manually enable functionality.

Based on a glance at the Rust setup module, it seems like user simply needs to set a persistent environment variable for VOLTA_HOME, and extend their PATH with $VOLTA_HOME/bin. (Did I overlook something?)

It would be helpful to include the above information in that section, and inform the user that setting them in advance of the installation will customize the install root.

---

Further, it would be useful to explicitly document the FS roots for all mutations which occur during installation, so that a cautious user who can't read the Rust source can be made aware and act accordingly. This seems partially in place (see Windows Installer and Unix Installer), but the text in those descriptions aren't comprehensive of all mutations (e.g. the shims directory on each platform is not included in the listed paths).