[BUG] The uncanny afterlife of EXIST_HOME

[duncdrum]

Describe the bug
$EXIST_HOME used to be meaningful before v5, hence, it frequently appears in the docs. However, as a way to locate exists home directory on a given environment it is no longer useful. We need to come up with a better way of saying exist's home folder copy pasting from the docs no longer works as it did.

Expected behavior
Code samples from the docs should work.

Context (please always complete the following information):

• eXist-db Version: [e.g. 5.0.0 and up]
• App Version: [e.g. 5.0.0 and up]

Additional context

• all of them
• supersedes backup.xml: Missing information about a configuration aspect #308