Adds info about custom variable types and their scopes

[agentolivia]

Resolves #448

• Adds a table describing custom variable types and their scopes.
• Adds a troubleshooting section to the end with a tip about rebuilding previews.

To test:

• Go here and review the new table under the heading "Custom variable scopes".
• On that same page, scroll down to the bottom of the page and review the new Troubleshooting section.

[agentolivia]

@q0rban Added steps to test in the description. This is ready for review. Thanks!