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

[bearomorphism]

#1640 (comment)

Historical note

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

Action items

• 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

[bearomorphism]

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

[2509abhi]

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

[bearomorphism]

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

[bearomorphism]

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