Document some of the common lsp-mode shortcuts

To this page: https://erlang-ls.github.io/editors/emacs/ Or at least link to https://github.com/emacs-lsp/lsp-mode#commands

Probably some other tools that a powered by lsp-mode as well (helm-lsp, imenu etc)

That's because, if you don't have experience with lsp-mode, you don't really understand what you can do / which features are now available when you are done with installation.