Docs: Update dialog(modal) documentation

[saad-mo]

Modal will be renamed to dialog and we need update the following pages on the documentation site:

• https://sap.github.io/fundamental/components/modal.html
• https://sap.github.io/fundamental/demo-pages/modal-overlay-demo-page.html

Related issues: #935, #977

[ritarora]

@saadmhybris As I have created the new Dialog component and we will be deprecating Modal, do we still want to update the documentation?