Improve technical documentation

[echebbi]

Motivation

Current technical documentation does not provide enough details about how the plug-in is architected, how events are handled, etc.

Explain such topics in depth would help possible contributors.

Proposed Solution

I just published a more detailed user documentation:

• https://discord-rich-presence-for-eclipse-ide.readthedocs.io/en/latest/customize/change-language-icons.html

The Contribute to the plug-in part should be improved with details about plug-in's internals.