docs: Add detailed document on PP Backend functionality, interactions…

… and required technologies (ADORSYS-GIS#44) ADORSYS-GIS#32

* docs: Add detailed document on PP Backend functionality, interactions, and required technologies

    In this commit, a comprehensive document is created, providing a thorough overview of the PP Backend's
    functionality and its interactions with other components. The document also includes detailed information
    on the technologies necessary for implementing the backend and also how it should be tested. This documentation
    will serve as a valuable resource for understanding and developing the PP Backend effectively.

    Signed-off-by: [Awambeng Rodrick] <awambengrodrick@gmail.com>

* docs: adding more info on how to implement some backend features

* docs: updated backend documentation

* docs: updated backend documentation

* Updated mermaid diagrams

* Update backend_guide.md