Add 'usage guide' documentation

[dvcrn]

A lot of questions could have been answered if proton had a proper usage documentation. We should add one at some point

Some ideas would be:

• Explain editor navigation, jumping between panes, etc
• Explain how to use the different input modes (vim-mode, vim-mode-plus, emacs, default)
• Explain the concept of ,, syntax specific leader
• Theme switch
• proton.core.post-init-timeout
• proton.core.wipeUserConfigs

[dvcrn]

I added instructions for setting up .proton. If there are other suggestions what to add into the manual, post it here.