Include njs into developer documentation

[pleshakov]

Currently, njs is not mentioned in the developers doc at all -- https://github.com/nginxinc/nginx-gateway-fabric/tree/main/docs/developer -- so for new developers, it is easy to start working on njs-related parts.
At the same time, we do have the documentation for it -- https://github.com/nginxinc/nginx-gateway-fabric/blob/9dc786a405ac505a16e7c098451fbbcd6a5c8395/internal/mode/static/nginx/modules/README.md

Acceptance criteria:

• njs-related documentation is easily accessible for new developers
• njs-related documentation is integrated with the existing developer docs

[mpstefan]

During our community meeting, we discussed:

• It makes sense for the readme to exist in the modules directory, but we need a link somewhere in the developer docs directory
• We could potentially make its own document which links out to the readme in the module directory
• Or new doc could be an intro to writing NJS then a link to the readme
• Adding a link to the contributing.md in the development section
• Updating the project structure to mention NJS