Import wiki’s CLI docs into USAGE.md

This commit takes all of the command-line options documentation from the
wiki [1] and merges it with the command-line options documentation
that’s in USAGE.md. This change will eventually allow us to remove the
command-line options documentation from the wiki so that we only have
one list of command-line options instead of two.

Before this change, USAGE.md used a Markdown table and the wiki used an
HTML table. I ended up replacing the Markdown table in USAGE.md with an
HTML table because it was easier to copy and paste the HTML table from
the wiki than to convert the HTML table from the wiki into a Markdown
table.

The main motivation behind this change is to make it easier to create a
future commit. That future commit will add documentation for the
-additionaldir command-line option. Making sure that there’s only one
list of command-line options instead of two will make it so that
-additionaldir doesn’t need to be documented in two different places.

This is related DescentDevelopers#618, but it doesn’t fully fix the issue.

[1]: <https://github.com/DescentDevelopers/Descent3/wiki/Running-Descent-3#command-line-options>