update "how to add custom study area to landweb" in manual

[achubaty]

https://predictiveecology.github.io/LandWeb/modifying-landweb.html#example-2-adding-a-new-study-area

• per-study area LTHFC maps (temporary stopgap);
• how to allow different layers in LTHFC to be used via e.g., study area name (useful for evaluating different versions of the LTHFC map);
• direct user to add custom study name to the 'allowed' list at the top of InitMaps();