docs: improve documentation on resolving EACCES

[mohd-akram]

Simplify the steps a bit:

• Backing up the computer and creating the directory is not necessary, since it's created automatically by npm.
• Use the ~/.local directory which is the standard place for user-specific software, and likely to already be in the user's PATH.
• export is not needed since PATH is already exported.
• Add a note for zsh which doesn't use ~/.profile.
• Use a different package as an example, since jshint is not regularly updated, and linters should generally be in devDependencies and used through npm scripts.
• Ensure the user can run the installed tool.
• Remove mention of NPM_CONFIG_PREFIX which is not sufficient, since the PATH still needs to be updated to use global tools.