Improve documentation to explain more how to register custom LSP command.

[angelozerr]

Discussed in #363

^{Originally posted by CppCXY June 18, 2024}

My own language server supports the CodeLens feature, but it needs to use the client's command to implement the click to pop up usages. How can I register such a command and pop up the usage UI?

It exists a documentation at https://github.com/redhat-developer/lsp4ij/blob/main/docs/DeveloperGuide.md#lsp-commands but it would be nice to add a sample piece of code.