Complete dev environment documentation

[IvanM26]

The following changes have been applied to README:

• Added instructions that explain how to close and delete the dev environment
• Changed README images location
• Added missing instruction to populate dev database
• Added comment about pgAdmin not working in all browsers
• Replaced "pgadmin" with "pgAdmin" accordingly
• Added content about how the Dev Container set up finishes when no error occurs and how to open an R Terminal
• Added system requirement for gert R package
• Made an adjustment to an internal README link

Closes #108