Rename the option cz bump --files-only to --version-files-only

Historical note

The config option version_files was called files long long time ago.

Add an option --version-files-only to cz bump
Mark --files-only as deprecated and will be removed in v5
Check whether there is a configuration file option called files_only. If so, do similar things to configuration options.
Update doc

@Lee-W

@Lee-W I think this can be a good first issue

Hi! I’d like to take this issue #1658. Let me know if that’s okay.

I am working on #1657 which is making a tremendous change on documentation structure. You can make the documentation change after it. thx

I saw your changes. I guess there will not be conflicts hopefully (if there are conflicts I will fix it on my branch)
nvmd