Petascope - add technical document for developers

[Bang Pham Huu]

• an example of changing petascopedb schema with liquibase on ​https://doc.rasdaman.org/03_contributing.html#petascope-developer-s-documentation
• when changing the wcps ANTLR4 parser
• when adding a new endpoint (controller) like objectExists

An up to date documentation of petascope has to be established, for continued maintenance in future. Contents:

• architecture overview
  • user API (OGC services plus proprietary external APIs)
  • characterization of the system layers
• UML class diagrams
• database schema
• configuration files used